Migrated from Trixbox --> FreePBX. Now can't hear ring tone when placing calls

I manually migrated a config from a Trixbox to the latest version of FreePBX on another server. Now, some of my remote users (Polycom 501 handsets) aren’t able to hear an ringing indicator when placing outgoing calls. The call is connected ok, it’s just the missing tone.

Does anyone have any suggestions? Thanks!

Ok. The problem is that music-on-hold is being played rather than the default ringback tone. How do I change this?