Were you ever able to resolve this? If so please come back to this post to provide an update so others can benefit from the solution. You can disable internal voicemail with the no voice mail internal command. There are some other hooks into voicemail like extension 8500 and the *98 SPRE that would need to be accounted for and MWI would likely not work on the IP Phone. Also, the stutter dial tone may not work either even in an SLA type setup. This isn't a supported application for the NetVanta 7100, but if you had any luck please let us know.
No i gave up, retrained the customer, thanks anyway everyone