Do the provide global DNS on their DNS servers? (I don’t think the standard installation of FreePBX supports a configuration where you get most of your DNS from the public internet and certain domains from a closed system. You will need to use their DNS exclusively, or you will need to run your own forwarding domain name server, locally.)
Is du.ae appended to .fmc, or is their server on a different top level domain?
Does the “sbc” mean it needs to be treated as an outbound proxy?
From what addresses do incoming calls arrive?
Does FreePBX need to make calls on, or receive them from, the public internet? (Requires relatively complex NAT configuration.)
Do they accept caller ID on outgoing calls, and if so how?
Please confirm that no registration is required (seems likely, but need to be sure).
Technically, your eth0 should be /32. It’s on subnet 172.20.0/21 and eth1 is on 10.15.34.48/30