Hi Daniel,
Cisco Unified Communications Manager 8.0(3) and later enables you to configure a queue with Call
Arrival Mode to Hold Queued Calls when it arrives on the CTI Port. This function places calls on
hold so that Music on Hold can be played to the caller while they wait for an operator to answer.
If you use this mode the call is charged from the time that it is answered and put on hold on the CT
Gateway.
The custom MOH file uploaded on cucm needs to be applied to the CTI ports for CUAC. Please check page 89 of the CUAC admin and install guide below
http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/cucmac/cuaca/10_5_1/install_admin_guide/CUACA1051WAG.pdf
HTH
Manish