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SIP Response Codes: A Complete Guide

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Learn about SIP response codes, how they function, and the different types of response codes available. Understanding SIP codes can help you identify issues within your communication system.

What are SIP Response Codes?

Session Initiation Protocol (SIP) is a signaling protocol used to facilitate and control communication sessions. As such, SIP lets users make and receive calls over the internet instead of traditional phone lines. This paves way for unified communications by enabling the transmission and sharing of voice, video, and other files.

A SIP session is based on a request/response transaction. Therefore, each session consists of a SIP request and at least one SIP response. Response codes indicate the status of the SIP request when making a connection between two or more parties.

How Do SIP Response Codes Work?

SIP responses use a 3-digit response code to outline or detail the status of a SIP request. For example, was the SIP request accepted, was it a bad request, and so on. These codes are divided into 6 broad categories, namely:

  1. Informational/Provisional
  2. Success
  3. Redirection
  4. Client error/Request failures
  5. Server error
  6. Global failure/error

These codes also contain a “reason phrase” which can be varied to provide additional information or in a different language.

Different Types of SIP Response Codes

So, what are the different types of SIP response codes and what do they indicate? Important abbreviations to be aware of:

  • User Agent Client (UAC) – initiates the requests
  • User Agent Server (UAS) – responds to the requests
  • Uniform Resource Identifier (URI) – a string of characters that unambiguously identify a particular resource

Here we will look at each response code in each category in detail:

1xx = Informational SIP Responses

1xx SIP response codes are sent at any time when a connection between two parties is being created. Common 1xx codes are:

100 – Trying: The request was received and an extended search or unspecified action is being performed.

180 – Ringing: The user agent has received an INVITE (SIP request code) and is alerting the user.

181 – Call is Being Forwarded: The call is being forwarded to another destination, receiver, endpoint.

182 – Queued: Indicates that the destination is temporarily unavailable and the server has placed the call in queue.

183 – Session Progress: Provides information about the progress of the call.

199 – Early Dialog Terminated: Indicates that an early dialogue has been terminated. Usually sent by the User Agent Server.

2xx = Success Responses

2xx codes indicate that the SIP request was received, understood, and accepted. Common 2xx codes are:

200 – OK: Indicates that the request was successful.

202 – Accepted: Indicates that UAS has received and accepted the request, but it has not been authorized or processed by the server yet.

204 – No Notification: Indicates that the request was successful. However, no response will be received.

3xx = Redirection Responses

3xx response codes inform the UAC about redirections and further action is needed to complete the request or reach the UAS.

300 – Multiple Choices: The request address returned several choices with different locations. The UA can select one of several options of endpoints to redirect the request.

301 – Moved Permanently: The user is no longer at the address used in the request. The original request URI is no longer valid. A new address will be provided in the Contact header field. This address should be saved and used in the future.

302 – Moved Temporarily: A new address will be provided in the Contact header field. The UAC should try the new address. This address should not be saved for the future.

305 – Use Proxy: To access the destination and address, a proxy is required. The proxy will be displayed in the Contact field.

380 – Alternative Service: The call failed, but alternatives are noted in the message body.

4xx = Request Failures/Client Error

4xx response codes indicate that the message was not processed due to an error. The request may include bad syntax and therefore cannot be fulfilled at this server

400 – Bad Request: Indicates that the request could not be understood.

401 – Not Authorized/Unauthorized: Indicates that the request requires user authentication.

403 – Forbidden: Indicates that the server is refusing to fulfill the request, even though it has understood it.

404 – Not Found: The user does not exist in that particular domain.

405 – Method Not Allowed: The method specified in the Request-Line is understood, however, it is not allowed.

406 – Not Acceptable: The resource can only generate responses with unacceptable content.

407 – Proxy Authentication Required: Similar to the 401 code, the request requires user authentication.

408 – Request Timeout: The server couldn’t find the user within a suitable time frame.

409 – Conflict: User already registered (deprecated).

410 – Gone: The user is not available here anymore.

411 – Length Required: The server needs a valid content length before accepting the request.

412 – Conditional Request Failed: The given precondition has not been met.

413 – Request Entity Too Large: Indicates that the request message body is too large.

414 – Request URI Too Long: The server refuses to accept the request. This is because the request URI is longer than the server can interpret or understand.

415 – Unsupported Media Type: Requested message body is in a format that is not supported by the server.

416 – Unsupported URI Scheme: The request URI is unknown to the server or not supported by the server.

417 – Unknown Resource-Priority: Indicates that a resource-priority option tag was present, but without a Resource-Priority header.

420 – Bad Extension: Bad SIP Extension was used. The SIP extension is not understood by the server.

421 – Extension Required: The server requires a specific SIP extension that is not listed in the supported header.

422 – Session Interval Too Small: The request contains a Session-Expires header field with a duration or interval that is too small or below the minimum.

423 – Interval Too Brief: Similar to 422, the expiration time of the resource is too short.

424 – Bad Location Information: The request’s location content was unsatisfactory or “bad.”

428 – Use Identity Header: An Identity header field is required by the server policy and one has not been provided.

429 – Provide Referrer Identity: The server has not received a valid Referred-By token on the request.

430 – Flow Failed: A specific “flow” that was sent to a user agent has failed. However, other flows may succeed.

433 – Anonymity Disallowed: The request was rejected since it was anonymous.

436 – Bad Identity Info: The request has an Identity-Info header filed and the URI contained cannot be identified.

437 – Unsupported Certificate: The server could not validate a certificate for the domain that signed or sent out the request.

438 – Invalid Identity Header: Server obtained a valid certificate used to sign a request. However, the server could not verify the signature.

439 – First Hop Lacks Outbound Support: The first outbound proxy doesn’t support the “outbound” feature.

440 – Max-Breadth Exceeded: A client that received a 440 response can interpret that its request did not reach all possible destinations.

469 – Bad Info Package: A 469 response indicates that the receiver is not willing to accept this Info Package.

470 – Consent Needed: The source of the request did not have the recipient’s permission to make such a request.

480 – Temporarily Unavailable: The recipient is currently unavailable.

481 – Call/Transaction Does Not Exist: The server received a request that does not match any dialogue or transaction.

482 – Loop Detected: Server has detected a loop.

483 – Too Many Hops: Max-Forwards header has reached the value ‘0.’

484 – Address Incomplete: The requested URI is incomplete.

485 – Ambiguous: The request-URI is ambiguous.

486 – Busy Here: The recipient is busy.

487 – Request Terminated: Request has terminated or canceled.

488 – Not Acceptable Here: Parts of the session description of the request URI are not acceptable.

489 – Bad Event: The server could not understand an event package specified in an Event header field.

491 – Request Pending: Server has some pending requests from the same dialogue.

493 – Undecipherable: The request contains an encrypted MIME body, which the recipient can not decrypt.

494 – Security Agreement Required: The server has received a request that needs a negotiated security agreement.

5xx = Server Errors

5xx response codes indicate that there’s an issue with the server and it has, therefore, failed to fulfill a valid request.

500 – Server Internal Error: The request could not be fulfilled due to some unexpected condition.

501 – Not Implemented: The SIP request method is not implemented here.

502 – Bad Gateway: An invalid response was received from a downstream server while trying to fulfill a request.

503 – Service Unavailable: The server is in maintenance or temporarily overloaded. Therefore, cannot process the request.

504 – Server Time-out: The server tried to access another server while attempting to process a request. However, there was no timely response.

505 – Version Not Supported: The SIP protocol version in the request is not supported by the server.

513 – Message Too Large: The length of the request message is longer than the server can process.

555 – Push Notification Service Not Supported: The server does not support the push notification specified in the SIP URI parameter.

580 – Precondition Failure: The server is unable or unwilling to meet the constraints specified in the request.

6xx = Global Failures/ Global Error

The request cannot be completed at any server.

600 – Busy Everywhere: All possible destinations are busy.

603 – Decline: Destination cannot participate in the call and there are no alternative destinations.

604 – Does Not Exist Anywhere: The requested user does not exist anywhere.

606 – Not Acceptable: The user’s agent was contacted successfully. However, certain aspects of the session description are not acceptable.

607 – Unwanted: The call is unwanted by the recipient. Future attempts are likely to be similarly rejected.

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Related: SIP Trunk Pricing Breakdown (2020)

SIP Line vs. SIP Trunk: What’s the Difference?

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Many businesses offering local and international services often wonder if there is any way to modernize and enhance communications without receiving overwhelming bills each month. Well, SIP trunking and SIP lines make a fine solution. Let’s look at the difference between a SIP trunk and line so you can get the phone system that works best for you.

SIP Line vs SIP Trunk: Understanding the Difference

Oftentimes the terms SIP line and SIP trunk are used interchangeably. However, there are two components that work together and are not the same thing. Here we explain what each term entails so you have a better understanding.

What is a SIP Line?

SIP lines are also called SIP channels or SIP sessions. A SIP line is an element of a SIP trunk through which data can be exchanged between two points. A SIP trunk holds large numbers of SIP lines or channels. During calls, a SIP channel is the unit’s capacity to support an incoming or outgoing call. In other words, each call uses one channel.

And so, companies needing SIP lines are more than satisfied by one SIP trunk. However, exactly how many lines your business needs depends on the number of concurrent or simultaneous calls you handle per day. SIP providers offer a variety of plans:

  • Unlimited channels
  • Unlimited channels with a fixed number of session minutes
  • Fixed number of channels with unlimited minutes
  • Multiple trunks for multiple offices on the same network

What is a SIP Trunk?

SIP trunks connect your business’ private branch exchange or PBX system to the internet. In other words, it converts your traditional phone system into a virtual or digital version.

As noted above, a SIP trunk holds SIP channels and makes concurrent SIP calling possible. In VoIP phone systems, a SIP trunk can hold about a hundred SIP channels. These lines can be distributed across different departments, devices, and so on.

SIP trunking enables your business to make and receive calls over the internet instead of a traditional POTS line. This way, this method of communication supports both traditional as well as VoIP phone systems. With SIP trunking, you can transmit voice, video, text, and other unified communications.

Benefits of SIP Trunking

Why should your business consider SIP trunking as part of your communication system? Here are some ways SIP trunking benefits businesses:

1. Unified Communications: With SIP trunking, you can handle voice, video, and text transmission through one platform. Additional services are not required.

2. Low Cost and Set-up: SIP trunks don’t need extensive hardware or software to function. A reliable internet connection is more than enough.

3. International Coverage: With SIP trunking, your local and international business calling bills reduce tremendously.

4. Scalability: You can always add extra SIP lines to your SIP trunk if/ when needed. Again, there is no extensive set-up or installation required.

5. Tools for Better Customer Service: By providing voice, video, and text communication, you can enhance customer support by adding video conferencing, text messaging, and file-sharing.

Does Your Business Need a SIP Line or SIP Trunk?

The decision really isn’t one or the other. This is because SIP channels work within a SIP trunk. The real question is, how many channels does your business need?

A SIP trunk can host multiple SIP lines, allowing for multiple concurrent calls to occur simultaneously. In other words, if your business has 250 SIP lines, it can handle 250 calls at the same time. Each call takes up 1 SIP line.

Similarly, SIP trunking prices or monthly fees will depend on how many lines your SIP trunk holds. To determine this, you will need to know how many incoming and outgoing calls occur during a day. Work with your sales, marketing, and customer service teams to get an approximate number.

If your business does not receive or make a large number of calls per day, then you may choose to have a few employees or small teams on each SIP line. Additional SIP lines can be added later.

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What is Session Initiation Protocol (SIP)?

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In this comprehensive post, we will explain Session Initiation Protocol (SIP) and its important features.

What is Session Initiation Protocol?

Session Initiation Protocol (SIP) is an internet signaling protocol that is used to initiate, maintain, and terminate real-time voice and video communication. An internet protocol is a set of rules for routing packets of data across a network. SIP is often used for mobile phone calling over LTE and in IP telephony systems.

It enables businesses to take their communications to the next level, by enabling not just voice-sharing but video conferencing and text messaging as well. Unified communications are made possible for an organization through SIP.

Important SIP Features

Note that SIP is a signaling protocol. In other words, it doesn’t handle the specifics of multimedia sessions. Other protocols such as the Sessions Description Protocol and Real-Time Transport Protocol handle media details and data delivery respectively.

Session Initiation Protocol initiates a call between two or more people where the SIP client establishes specifics of the request. The other party can choose to accept or reject it. This transmission is done securely through Transport Layer Security (TLS).

Additionally, Session Initiation Protocol makes it possible for users with different service providers to interact and communicate. It can boost your IP telephony call with a process called SIP trunking.

Session Initiation Protocol versus Voice over IP

SIP and VoIP are often used interchangeably and therefore, it is important to understand the difference between these two types of technology. See below:

SIP Trunking Voice over IP Tech
Signaling protocol within VoIP Family of technologies related to communications
Multimedia transmission Only voice messages transmitted
Requires only a modem VoIP devices need to be connected to a computer
Uses a peer-to-peer system to handle large amounts of data Uses a central network to organize and transmit traffic

Benefits of Session Initiation Protocol

Why do businesses benefit from SIP trunking and why should you consider this service for your communication system? It’s simple: SIP and SIP trunking can boost and enhance communications in a multitude of ways. It is not only cost-effective but provides phone solutions to improve internal and external communication. Here are some of the top benefits of SIP trunking.

A HIghly Cost-Effective Service
SIP calls are routed over the internet and by doing so, this gives you the ability to contact local and long-distance contacts for cheaper calling rates. Pricing for a SIP trunking service is pretty straightforward. You sign up for a monthly subscription and your bill remains predictable. Furthermore, most SIP providers do not require you to sign long-term contracts. Check out our detailed analysis of SIP trunking prices for your information.

Tools to Expand Internationally
Part of growing your business globally involves creating strong communication channels between your company and its customers. SIP trunking paired with virtual phone numbers is a great solution for businesses looking to expand their reach into international markets. You can reduce your calling costs while providing convenient (and even free) ways for customers to connect with you through local numbers and toll free service.

One Platform for All Communications
You can transmit voice, video, text, and more via SIP channels. This technology makes multimedia communication possible from one platform, instead of having it spread across 4-5 different services or software. This can help increase office productivity and reduce costs that can build up from different platforms.

More Session Initiation Protocol Resources

Contact us at 1 (877) 898 8646 if you want to learn more about Session Initiation Protocol and let an expert help you through the process!