Another thing you should test, is how many calls can be made to the Man number before you get a busy. I had 60 sites that when we move them and port the old main number to the new location, some could only get 3 call before you got a busy and some got 10 calls before the busy. Found out some Telco's give you only 3 CALL PATHS for free and some give 10 for free but any more then the 3 or 10 and you had to pay extra. So I would test it.
This is a Signature and not part of the answer, it appears on every reply.
This is an Analogy so don't take it personally as some have.
Why change the engine if all you need is to change the spark plugs.