I hope this FAQ will help with enquiries about setting two sites for VoIP. The link could be xDSL, lease lines or even Frame Relay controlled by third party routers. It does not matter, as long as you could ping from one network to another, and that the third party routers support the protocols that you are using. Other configuration with the WAN ports are possible.
I do not intend to rewrite the manual, but if my information is wrong, please do correct me. Please note I have several fully functional sites with this similar setup.
Required hardware: IP Office or ArgentBranch at both sites VCM at both sites. Static IP Address for ADSL modems.