I thought a problem like this was described here shortly after iOS 15 was released, but I searched extensively and couldn't find it. So, I'll try again and hope someone has some advice or solution for this problem.
My wife and I each have an iPhone on our T-Mobile account. About a week ago, we discovered that no one could call either of our phones. All calls were (and still are) routed directly to voice mail. We receive no missed call notifications, although we will get the voice mail if one is left.
Do Not Disturb is definitely off. I think my wife upgraded her iPhone 11 Pro Max to iOS 15 about that time, but I've not upgraded my iPhone 8 yet. We can make outgoing calls, although it takes about 30 seconds before it will attempt the call. Interestingly, this problem only occurs when we're at home. If we travel to another location (and another cell tower, I presume) then calls come in normally.
We've called T-Mobile twice and put in a ticket, but so far, the problem still exists. Has anyone else experienced something like this? If so, how did you solve it?