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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.
- Set up an unmanaged gateway on each PBX whose 'address' is the SIP domain name (not the fully qualified hostname) of the other PBX.
- Setup a dial plan to direct calls to the other PBX - use the 'site-to-site' dial plan type.