Hi Tom ,
Are you using a Partner II system by chance? If so, and the voice mail card is in the second cabinet, the expansion processor needs to be R4 or greater. The main processor must be R4.0 or greater also. The voice mail will not communicate with the system otherwise. This may also be true of the Partner + in it's single cabinet.
I just had a system install the other day where I had to put the voice mail (VS R5.0) in the first cabinet on a Partner II R4.1. The expansion cabinet was at R3.1 and I had the same fault. The vm ports gave a busy when called directly. The customer bought this from another vendor.
-Chris