SIP Invitation

A successful SIP invitation consists of two requests.

INVITE followed by ACK.

The INVITE request asks the callee to
join a particular conference or
establish a two-party conversation.

caller confirms that it has received that response by sending an ACK request.

If the caller no longer wants to participate in the call, it sends a BYE request instead of an ACK.

1, the proxy server accepts the INVITE request (step 1),
contacts the location service with all or parts of the address (step 2)
and obtains a more precise location (step 3).
The proxy server then issues a SIP INVITE request to the address(es) returned by the location service (step 4).
The user agent server alerts the user (step 5)
and returns a success indication to the proxy server (step6).
The proxy server then returns the success result to the original caller (step 7).
The receipt of this message is confirmed by the caller using an ACK request, which is forwarded to the callee (steps8 and 9).
Note that an ACK can also be sent directly to the callee, bypassing the proxy. All requests and responses have the same Call-ID.