SIP call flow is the sequence of steps involved in establishing, maintaining, and terminating a Session Initiation Protocol (SIP) call. SIP call flow involves several steps such as registration, session establishment, media negotiation, and call termination. The call flow typically starts with a SIP client sending a REGISTER request to a SIP server to register its address. To initiate a call, the caller sends an INVITE request to the callee, which includes the desired media parameters. The callee responds with a 200 OK message, indicating acceptance of the call. Media negotiation takes place through SDP (Session Description Protocol) exchange, where the caller and callee agree on codecs, IP addresses, and ports. During the call, SIP messages like ACK, BYE, and UPDATE may be exchanged for call control purposes. Finally, the call is terminated by sending a BYE request from either the caller or the callee. SIP call flow can be influenced by various factors like network conditions, firewalls, and NAT traversal.