Reprogramming MAY be required, but it shouldn't be necessary, IF Comcast did their homework. Obviously they didn't. I suggest you go back to a REAL phone company, AND call a Mitel tech to sort out the problems.
Either that or tell your Comcast rep that he had better give you a written letter saying that Comcast will reimburse you all costs for the Mitel tech and lost business, SIGNED BY HIS/HER SUPERVISOR. You won't get it, and that will hopefully point out the folly of not using a REAL phone company, supervised by the F.C.C. frown John C.

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.