Bad form to reply to one's own post but....

With some help from another reader we established that it *can* work
provided that both ends can solve going through NAT. My Zyxel is OK as
it can embed the external IP address in the VOIP packet - but the
other end was not. So my end was trying to reply to his *internal*
address as he normally went through a STUN server. So we ended up with
a one way conversation.

Still no idea why my Line 1 can't call Line 2 though. Snooping the
packets it appears that they are just throwing invites at each other.
Anyone know where I can find the gory details of VOIP packets?

--
Regards
Dave Saville

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