[Asterisk-Users] One-way audio with VPN

[Alex Lake]

I've got a one-way audio problem, but I've looked through a few 
documents on the subject and I'm not sure that it's the same issue.

User A calls a local Asterisk user B via a public SIP gateway 
(voiptalk.org) using (sip:110 at siptest.dmclub.net)

B is connected to the Asterisk server via VPN

B is registered (and has successful bi-directional conversations with 
other users on the VPN)

Asterisk correctly forwards the call via SIP and B's phone rings and is 
answered, but B can't hear A

So there appears to be an audio-path blockage from A via Asterisk to B.

Now if A leaves a voicemail message on the asterisk box, that's fine 
(the sound file contains a recording of A's voice!)

Therefore, it looks like the problem is to do with the forwarding of RTP 
packets by Asterisk from A (Internet origin) to B (VPN).

Any ideas?