One way audio on LAN, fine through NAT

Hi all,

I have a strange problem with a 1.6 asterisk here.
It’s registering one peer with sipgate which works fine in both directions (*) and now we have a small Auerswald VoIP-aware PABX which should forward some legacy ISDN calls and bridge analog equipment to the asterisk server. It’s a test how those PABXs could be integrated into an asterisk environment if it is already installed.
A Snom 320 VoIP phone registers fine with the peer on the PABX, but when I register asterisk to it, I get one way audio.
I know this problem from NATted external channels, but this one is on one single LAN segment and subnet!

Any idea?

(*) today the asterisk failed registering to the Sipgate peer, telling me:
WARNING[2789]: chan_sip.c:18165 handle_response_register: Forbidden - wrong password on authentication for REGISTER for ‘105xxxxe18’ to ‘

A wireshark dumpm shows "SIP/2.0 403 Forbidden (No valid username)"
The username in the sip.conf is the same a VoIP telephone registers just fine with.