What is "SIP fork" in the context of call processing?

In the context of call processing, a "SIP fork" refers to the process of sending a SIP INVITE request to multiple endpoints simultaneously. This is a fundamental feature of the Session Initiation Protocol (SIP) that enables more flexible call routing.

When a SIP INVITE is forked, it can be sent to multiple contacts that may be associated with a single user or service. For example, if a user has multiple devices registered (like a desk phone, softphone, and mobile app), the INVITE can be sent to all of them at once. As a result, multiple rings can occur, allowing the user to answer the call on any of their devices. This forking capability is crucial for ensuring that calls reach users regardless of which device they choose to use at that moment.

The other options, while related to SIP, do not accurately represent the concept of SIP forking. Ending a session prematurely is a different operation altogether, transferring calls pertains to managing established sessions, and blocking unwanted calls refers to security and filtering, none of which capture the essence of a SIP fork.