asterisk users - Sep 2005 - Connect not signalled (SIP -> Zap)

Hi,

I've had a strange problem several times during the last days:
A call is established, both parties have audio in both directions,
but asterisk is still waiting for connect.

Thus after timeout (120secs) the call is terminated with either
busy or no answer.

This is annoying for the both parties, who are already speaking,
because they get interrupted.

In the cdr I can find the call with 120 secs duration, 0 secs billsec.


Has anyone had a similar problem so far?
Or any ideas?
Roger.