I had a similar problem yesterday in Chicago, however in my case I believe it was because several years ago I went ahead and manually switched to the new voicemail system when they were converting. To do this I believe that I had to change both the number that my phone called to get to VM and the numbers that unanswered calls were forwarded to.
Changing the VM number I call took care of getting me into VM; a quick call to T-Mobile got it cleared up for people calling me.
I and callers were getting an error about the subscriber not being in service.