I would like to thank everyone for their helpful replies. As I now understand it we will have to deep-six the current controller and unless we want to replace the telephones we should consider only Avaya equipment. The configuration would be Partner ACS (R6 or R7) together with either PARTNER Voice Messaging Release 3.0 (PVM R3) Large or PARTNER Messaging Release 1.0 (PMR1). I will keep this in mind as I try to better understand the alternatives.

I was hoping that I would be able to find a document or documents which would explicate this subject including multi-vendor, justification, puts & takes, configuration and costs. Ever the dreamer!.

Thanks again.

Pete