asterisk users - Jan 2004 - Suggestions for debugging an IAX1 one way audio problem

Hi,

I occasionally experience a one way audio problem when using the IAX1 protocol 
with a service provider. If I call a DID number in by group provided by the 
supplier which is redirected to my box, I only get the audio path back from 
my box to the phone I used to dial the DID number. 

If I use IAX debug, or tcpdump I only see packets going from my box to the
provider when the one way audio problem manifests itself. 

I'm using the latest tarballs of asterisk, zaptel, and libpri:
(Asterisk 0.7.1, Zaptel 0.8.0, and Libpri 0.5.1)

I'm trying to determine if the problem is on my end or at the provider's
end.
If anyone has a few addtional things to try, I'd be happyto hear about them.

Steve.