Been tracking an issue I have had for a while with a Asterisk installation and the callers not getting a ringback tone when calling other SIP/IAX2 trunks.
We were using 2Talk for the IAX2 trunk connecting to Asterisk with SIP based phones/soft phones.
To get the ringback tones working I just had to add these two rules into the Asterisk SIP Settings page in Freepbx.
Right at the bottom you can add custom rules, you need to add:
After making these changes and applying the config all phones now have ringback tone as they should.