How to reduce echo between internal extensions?

I have just setup two Android phones as extensions of my Freepbx. Both work great for incoming calls (havent tried outbound with this setup), but when I try to dial from one extension to another I get both parties hearing their own voices echo. Any idea what might be causing this and how to fix it?

Is NAT set to Yes in the extension setup ?

Yes, NAT is set to “yes” in both extensions setup.

Set it to “NO” and report your findings.