A telephone call on a VOIP, Voice Over Internet Protocol, System will rely on a SIP, Session Initiation Protocol to establish the connection between the 2 or more, calling parties.
A Basic Sip Session will look as follows,
The calling device (Desk phone, Computer, etc) send a Invite to the destination (person you are calling).
If the device is available and able to accept a call (Answers the call) , it will send a 200 OK back.
The calling device will send a ACK (Acknowledgement). The RTP Media stream will then begin ( Voice i.e. your conversation and or Video) and continue until the call is finished.
At which time the parting hanging up first, will send a BYE. The other party will send a 200 OK back.
This will terminate the session.
If a BYE is not received, or acknowledged with a 200 OK, even though both parties have hung up, their phones respectively. The session may remain on the Voice servers or PBX, consuming license or lines into the business.
A call trace of this conversation would look like below,
Tom calls Joe
Tom ---> Invite sip:firstname.lastname@example.org
Joe ---> to Tom 200 ok
Tom ---> ACK ---> to Joe
Media stream begins < ------->
Tom hangs up ---- BYE ---> sent to Joe
Joe sends 200 OK ---> to Tom
Call is ended