Yes you're right, when you're dealing with this of kit you're best to pay the extra $$ and get the professional convertor.

However, in the meantime, you can still go ahead with the Mute input to Empeg stuff - I'd attach a small on/off switch between your cars (empegs) +12v source and the cellphone mute - that way you can 'simulate' a incoming call simply by flipping the switch on - that lets you get the empeg side sorted - which will take a little doing, and then when your convertor arrives you can then wire it in, instead of the switch.

But until the convertor arrives you'd only have to flip the switch to be able to tell the empeg to switch to Cell phone mode, and when its unflipped, the empeg would be able to resume normal programming as it were.

You may decide that thats enough to be going on with and not bother with the convertor at all [or at least postpone it until you want it].

Does the Onstar system trigger the cellphone mute on outgoing calls too or only on incoming?