Hi everyone,
Ok, we've worked this one out. It was NAT after all.
Depending on the between which devices you were making a call, determined between which devices RTP packets were flowing.
Between VPN client to VPN client
- RTP packets directly. Call successful.
Between VPN client to Jabber on internet desktop
- RTP packets directly. Calls unsuccessful. Added no NAT to internal host to VPN clients. Calls successful.
Between VPN client and desktop phone
- RTP packets between MTP and VPN client. Calls unsuccessful. Added no NAT to MTP address to VPN client. Calls successful.
A tricky find but we got there! As a network engineer, before you go to sleep at night you need to ask yourself "Did I remember to turn the NAT off?"
Karl.