I hate to bring bad news:) However, looks like there is a bug in the ring groups module.
Very simple setup, nothing special in the hardware, no incompatibility issues — nothing.
Add sip trunk, add extension (or 2, 3 as you like). Create a ring group (I used default 600), set moh to default and add extensions to it,
Create inbound route to ring on the ring group 600, you can set a timeout to go to voicemail or anything else, doesn’t matter - it doesn’t work anyway:)
Make an inbound call from trunk, no music while ringing and it doen’t go to voicemail as well, just continues to ring…
Add the second trunk. Make a call using a second trunk - no music - just silence, however, it goes to voicemail.
I used differnt codecs combination, made numerous changes - same behaviour.
If I use a custom code, It works fine, music plays while extensions are ringing and it goes to voicemail as requested.
P.S. Sorry, forgot to mention - the freepbx version is 2.3.1 and asterisk is 1.4.13, standard install