488 Not Acceptable with Grandstream Phone and Free PBX 2.10

If you got this kind of error then you should check the codec that used by the phone and what are the codec that were enabled from the server.

First you can view through the phone display and make sure the codec that phone used.

In Free PBX 2.10 version you can move to Settings ----> Asterix SIP settings -----> Audio Codecs

and make sure phones supported codecs are enabled.

This is almost issue with Codec.