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- Make sure each PBX can resolve pertinent DNS records for each other's domain.
- If the servers are behind NATs, make sure that you have enabled remote access as though you were setting up to allow remote workers. See Remote User NAT Traversal.
- Setup Set up an unmanaged gateway on each PBX whose 'address' is the DNS SIP Domain name domain name (not the fully qualified hostname) of the other PBX. If remote is behind NAT, use a SIP Trunk instead of unmanaged gateway, set remote address to SIP domain, set SBC to sipXbridge and click on Advanced settings to set the port to 5080 and transport to UDP.
- Setup a dial plan to direct calls to the other PBX - use the 'site-to-site' dial plan type.