I've had this problem before and it's so erratic. Everything works great for about a week or two, then boom, a wrench is thrown in. I have 3 extensions, 200, 201, and 202 that are all in the ring group of 600. I haven't changed anything, yet only 202 would ring, but never go to voicemail, now none of them ring. I even deleted the ring group, re created it and the same with the inbound. It's really driving me nuts as I can't figure out why it's doing this. I can make calls no problem, I can call each extension from one another.