Well we started using the basic Phone Manager a couple of months ago. We certainly haven't put it under any stress (yet!), but so far, it works fine on Terminal Server, with no noticeable drop in system performance at any stage. BTW, we have an average of 12 users on at any one time if that gives you some idea of what you might expect.
A seperate PC acts as a server to handle all the voicemail. I doubt this software could degrade the system either if it were also on the TS, but we wanted it seperated anyway.