Firewall blocking internal calls

[Silent Bob]

I have just setup a W52P using freepbx and endpoint manager, the freepbx server is in the DMZ.
It auto provisions just fine and the phone tends to work quite well, except for internal calls from the W52P. These seem to be block by the firewall, strangely it seems to be blocking incoming packets from internet to the phone. I am only getting a host name, which is something like “ezsetupsystema01d4ab5bf43cfsc”, no ip address appearing on the firewall and seems to using ports 11780 – 11790.
When dialling the extension will ring but there is no audio in either direction after pickup. Opening up those ports on the firewall resolves the issue, however I don’t want to open up ports from external sources into my network.
Strangely enough, calls from the W52P to external numbers, external to W52P and internal calls to the W52P work just fine, its only calls from the W52P to other internal numbers.
Does anybody know how I can fix this issue?