Discuss this with your Digitcom Project Coordinator.
Usually this is obtained from your IT partner (either an in-house IT person, or an outsourced IT Firm).
In most cases, yes. However, some special adaptive wiring may be required in order for the installing technician to connect it to your new system. Make sure you communicated your desire to connect your existing MOH device to the new system. Even sending a picture of it, or a video, could be extremely helpful. The technician can then make sure they have the correct wiring to connect it.
For clients with more than one location, extension is especially important. Discuss this with your Digitcom Project Coordinator. They will help you with what kind of extension range will be necessary.