Freepbx sonicwall iax freewordldialup

Hi All

i have following setup:

boxA <> SonicwallA <> Internet <> SonicwallB <> boxB

boxA = boxB = debian & freepbx, all latest version (stable)
boxA is sitting in a different net work (location) than boxB

boxA and boxB do have SIP Clients, which can talk to eachother whithin the same network. i have as well trunks (iax) to freeworlddialup, which can be used and which are working. means callback from the website or a independet softphone client can call BOTH asteris server behind their sonicwalls.
what i CANT do is: setting up a trunk (iax) between A & B. it always says its unreachable, although the communication to fwd is working. what am i missing? i used to a tool called iaxping, giving me results by oining fwd, but NOT if i ping the one of the static IP (A || B). i used as well a tool called unsniffer network analyzer to see whats going on, and im missing the packages after the POKE get sent out by the client (requester).

what am i missing? any hints?

thx in advance

best tom