I don't think this is a problem with IVM or MOH so I am posting it here with Axon.
My main external line is setup so that when it gets an incoming call it passes it through to the IVM. My IVM is set to give a general welcome message and ask the user to press 1 if they know the extension they want, otherwise after a few seconds it connects to one of the groups and sets about calling all the available phones. When the IVM itself is ringing the external line can hear the normal ringing sound but when the IVM transfers back to the group you hear a slight bip sound and then silence on the external line, but the phones internally continue as normal.
I decided to try testing the group by having the external line directly ring the group without the need for the IVM, and the music on hold played to the external line perfectly (so they weren't listening to silence), but as soon as it goes to the IVM first and then transfers back to the group there is nothing.
I also tried ringing the IVM from one of my internal lines to follow it through and here is where things get a little odd - when the IVM transfers back to the groups the internal line can hear the music on hold perfectly.
It seems wierd that the internal line can hear the music on hold when transferred by the IVM, but the external line can't. The firewall has all the necessicary ports opened as well.
I hope someone can give me an answer to this - or if there is no answer I hope NCH can find the bug and fix it.