I have a Trixbox 192.10.10.xx in our main office, all sip devices in the same office even thou they have 172.18.xx.xx ip addresses connect to it without any problems. All of our branch offices have 10.x.xx.xx IP addresses and sip devices from those branches cannot connect to us, they cannot register. What may be a problem, we have all the appropriate routes set up in our VPN. I can ping our trixbox server from any branch office no problem, but x-lite phones will not connect.
Help
Since you have VPN I dont think this would be the issue but can there be a posibilty that NAT is being used ? If so see: voip-info.org/wiki-Asterisk+sip+nat