Interesting application. How many phones? Keep in mind the "music" will be a unicast RTP stream between the controller and each phone, so that's going to consume controller resources and network bandwidth. Phones will use the system MOH source, so to keep it off the trunks you'd have to change the call coverage and/or zone settings for the trunks. Look at "Workgroup/Branch Office Music on Hold" in the on-line help to set that up; but even with that, I'm not 100% sure that will work.