asterisk users - Jan 2005 - grandstream sip phone calling Zap/1 on TDM20B rings and answers but not hear voice

I have a grandstream 101 that is calling an extension on Zap/1 of a TDM20B.
The grandstream 101 can call another grandstream 101 at a different 
extension- that works fine.
The two phones on TDM 20B can call each other.- no problem.When I call 
the TDM20B Zap/1
from the grandstream phone it rings - I answer and I dont hear any voice.

for the grandstream I have tried allow=all for the codes but made no 
difference.

Any ideas on what I am missing?

Thanks,

Jerry

Could you give us the output of the console when you try the call ?

That would help us to point you in the right direction.