(Not an English teacher, but I know a bit about Telecom)
terminate - Definition and pronunciation | Oxford Advanced Learners Dictionary at OxfordLearnersDictionaries.com
"to end" (the signal is not relayed further)
So (metaphorically) the call starts in A and ends in the extension of B, i.e. someone in B is the receiver. Not in the meaning that the call is "hung up", though.