You may need to reverse T and R on the MOH card feed, or even use an isolation transformer. There cannot be any DC voltage on the input to the MOH card. It will saturate the transformer on the card and no audio will be able to pass to the MITEL 'side' of the card. (TMI me thinks, just make sure there is no DC voltage on the output of the amplifier.)

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.