What are SIP methods - requests and responses?

SIP uses Methods / Requests and corresponding Responses to communicate and establish a call session. SIP (Session Initiation Protocol) is widely used in VoIP and SIP phone systems to manage voice communications. The protocol defines SIP methods and responses to ensure reliable call control between users and systems.

SIP Methods Diagram

Common Use Cases for SIP Methods

SIP methods are used by VoIP phone systems to manage everything from call setup to teardown. For example, a SIP INVITE is used when starting a voice call, while BYE is sent when the call ends. SIP phones and VoIP servers rely on these methods for signaling and control. Most VoIP providers and SIP-enabled PBX systems support these standard SIP requests to ensure compatibility across devices and networks.

SIP requests

There are fourteen SIP Request methods of which the first six are the most basic request / method types:

  • INVITE = Establishes a session
  • ACK = Confirms an INVITE request
  • BYE = Ends a session

  • CANCEL = Cancels establishing of a session

  • REGISTER = Communicates user location (host name, IP)

  • OPTIONS = Communicates information about the capabilities of the calling and receiving SIP phones

  • PRACK = Provisional Acknowledgement
  • SUBSCRIBE = Subscribes for Notification from the notifier
  • NOTIFY = Notifies the subscriber of a new event
  • PUBLISH = Publishes an event to the Server

  • INFO = Sends mid session information

  • REFER = Asks the recipient to issue call transfer

  • MESSAGE = Transports Instant Messages
  • UPDATE = Modifies the state of a session

SIP responses

SIP Requests are answered with SIP responses, of which there are six classes:

  • 1xx = Informational responses, such as 180 (ringing)

  • 2xx = Success responses

  • 3xx = Redirection responses

  • 4XX = Request failures

  • 5xx = Server errors

  • 6xx = Global failures

Further reading

Last Updated
This document was last updated on 4 July 2025
https://www.3cx.com/pbx/sip-methods/