IAX trunk does not detect answer from ZAP channel

Hi all,

I have two asterisk (1.4) boxes configured and running fine. I decided to connect both of them via IAX2 trunk so as to be able to forward certain calls from box “A” to box “B” and let them go out through box “B” Zap channel.

boxA ----- (IAX2)---- boxB (Zap)-----> PSTN

Zap channel works fine on box “B” if the call comes from any SIP device, but when the call comes through the IAX2 trunk with the other asterisk, although the call becomes established (and I can even here the voice), this only works for the “ringing time”, for box “A” does not receive the “answer” signal from the remote Zap (through the IAX2 trunk) and after “ringing time” it thikns that there has been no answer from the remote party and drops the call (although it had been working fine for a few seconds!).

Is there something in the “standard” configuration of the IAX trunk that I must know? (just in case it matters: both asterisk boxes are behind NAT):


and a similar config for “Box B” …

both boxes see each other as “registered” (4569/upd has been opened).

Thanks in advance.