If it's only the MoH itself and transfer is fine, it sounds like a network/SIP related problem with the audio stream when you send a SIP request for the music on hold. Do you use the recommended default SIP device capabilities number for your softphones? Should be 71.
Is the MiCollab Client using Teleworker? When the problem occurs is the client located inside your corporate voice VLAN or outside over the internet? If it's inside, do you have the same problem outside or the other way around? Does the problem occur with internal calls and/or external calls?
Doing a SIP capture of the problem on MBG if you're using Teleworker could show some light on this.