I have set up a small system 6 extensions and have a few odd issues.
If I call in or out to a mobile then call work as expected however voicemail does not work from outside calls, but when i dial out or in from land lines the phone call connects however there is no audio.
I have
allow=alaw&ulaw&G729a&G729&GSM&G726
in my peer details as I have a lynchsys PAP2 which uses G729a as its codex
Unless you have the no longer necessary g729 licenses from Digium, you will probably not be able to use g729, however you can easily set the PAP2 to safely use ulaw&alaw alone