If it is a large project, highly recommend an outside source to discuss the business objectives and then approach a low level design. There really is no magic software to tell you how to get from point A to point b. Every legacy pbx/voicemail system is different. Every data/WAN network is different. Migration paths have some similarities in the approach, but when it comes down to the actually engineering of the migration, this is all customized from the business objectives. Very rarely do I see a 1:1 migration with same features, same xyz, etc.
