[Serusers] Problems with PSTN and REINVITE

I appear to be having problems with REINVITE messages lately (I may have simply never before received them and there was ALWAYS a problem... I don't know for certain). When I dial certain numbers -- so far it's only been numbers over the PSTN going through likely Asterisk gateways, everything goes fine UNLESS I get a reinvite. At that point, audio stops (though the stream appears to remain open). My UA will send an INVITE, I'll hear the answer on the other side for about a second -- audio will be fine... then a REINVITE is sent from the remote side and audio stops completely while the stream remains active. Has anyone run across this before? Is there something special I should be looking for in my ser.cfg to handle REINVITES as they bounce along? Is it a problem, perhaps, with a swallowed ACK somewhere? I'm hoping someone can give me a clue or two to point me in the right direction. Thanks, N.