bshank is pointing you in the right direction. UCCX uses a consult transfer to send calls to the agent, so you're not going to hear ringback, you're going to hear the network MOH source as configured on UCCX's CTI port pool. Occasionally people do want ringback instead of music, but the way that's done is to take a recording of ringback and make that the MOH source for UCCX transfers.
Make sure that MOH works on the voice gateway in question. For any call through that gateway, in or out, place it on hold from an IP phone. If you do get music, you have a config difference somewhere between the phone source and the UCCX CTI port group source and you need to look in that direction to see what's wrong with your UCCX port config. If you get dead air, you have broken MOH service for that gateway, for instance UCM might be configured for multicast MOH and the gateway can't receive it (bad network config, max TTL/hop count set too low in UCM for that MOH source, etc). Switching to unicast MOH can be simpler if you have a smaller system and you don't have good familiarity with unicast. If you get periodic beeps, CUCM thinks there is no MOH service available to that gateway and you need to check your MRG/MRGL config.
