It can happen to a call that comes in over zap and is answered on one of the sip phones, or on a call that goes out over iax from one of the sip phone. There seems to be no patter / rhyme / reason.
Users will be mid conversation and suddenly the remote party can no longer hear them. If the remote party doesn’t hang up first, the audio may return.
Normally, I would assume a problem like this is being caused by a flakey VoIP service, but as I mentioned it can happen on a call that comes in over zap, where the only VoIP is on the local netowrk, between asterisk and the phones.
Last night I upgraded asterisk to see if it would fix the problem, but when i spoke to one of my users today they told me it had already happened.