Music on Hold is a feature that consists of streaming music and relies on the Cisco IP Voice Media Streaming Application.
If a device must access a MOH server , then it must be configured under its MRG/MRGL.
Cisco IP Voice Media Streaming Application also provides Software Annunciator, Software Conference Bridge and MTP resources. Once this service is started under the Control Center, all previous application are enabled.
Cisco’s recommandation is to not run the Cisco IP Voice Media Streaming service on a server where you perform already call-processing as it can require high CPU overhead.
A Maximum of 51 audio sources can be configured across the cluster (50 in the form of audio files and 1 fixed audio source(usually a soundcard inside the server)).
MOH servers msut register with Call Manager and they communicate with it using SCCP.