I have been chasing a problem with aastra phones and asterisk v 1.4.0 and 1.2.15 as well. You will start the call and it will connect, then i will get a warning message on the console stating that the system exceeded the maximum retries for a packet to the phone and the call will drop.
this is the message:
Feb 26 19:32:42 WARNING: chan_sip.c:1226 retrans_pkt: Maximum retries exceeded on transmission email@example.com for seqno 1845472325 (Critical Response)
Feb 26 19:32:42 WARNING: chan_sip.c:1243 retrans_pkt: Hanging up call firstname.lastname@example.org - no reply to our critical packet.
same message every single time this happens.