SIP is a relatively simple, text-based, human readable protocol that is now the standard de facto for VoIP signalling. The protocol though (in my opinion!) is a little tricky, where typically the tricks are: details . In this first post of a series of "Trip-wires for the SIP fans", I'll talk about CANCEL. The main concept is easy: a caller may decide to cancel a call before this is answered. To do this, it sends a CANCEL request to the called party. What's important to know is that: A CANCEL request relates to an INVITE request, and does not relate to the SIP dialog the request may have created (or will create). For this reason the To header tag must be the same as the INVITE request, even if meanwhile there's been a provisional response to the INVITE creating a dialog (e.g. a 180 with a tag in the To header). From RFC 3261, 9.1 : The following procedures are used to construct a CANCEL request. The Request-URI, Call-ID, To, the numeric part of ...