SIP (Session Initiation Protocol) is a protocol to establish media sessions over a network. In simpler words, this is the thing that makes the ring at the remote user, starts the call and terminates it when one of the two parties hangs up.
Linphone uses default port 5060 to send and receive SIP packets. This is higly recommended by SIP 's rfc to use port 5060. So don't change this unless you really know what you are doing.