I'm having this same issue. Three weeks now, unable to make or receive calls. SMS and data work fine. Wifi works. LTE works. When I switch to 3G or 2G there is no connection and T-Mobile doesn't appear in the list of networks. The SIM card works fine in another phone so it must be an issue with the device.
Tried everything on the software and settings side, flashed different ROMs, flashed different radios including stock. No change whatsoever. I completely disassembled and reassembled the phone. Nothing was loose or looked broken, all contacts seem to be making contact.
I do vaguely recall dropping the phone recently, so I guess that must have been it. The only other possible cause was that Google Play Services was auto-updated the day before I noticed the problem. Of course that is unlikely to be the cause but just mentioning it in case anyone else had this issue starting around Oct 23, 2015.
As much as I hate to do it, I guess I'll have to get a new phone. Just no other solution I can see unfortunately.
For anyone finding this posting through Google, currently there is no VoLTE (Voice over LTE) support in the Nexus 5 on T-Mobile. If that is available when you're reading this, then you might look into enabling VoLTE and seeing if you can make voice calls using that. Based on my reading it seems like VoLTE is a software update (perhaps requiring the provider to support the device as well), but the consensus seems to be that VoLTE on Nexus 5 on T-Mobile is unlikely to happen. But they support it on the Nexus 6, so maybe there is some way it can be done on the Nexus 5. Also Wifi Calling on T-Mobile is not available for the Nexus 5. The consensus there is that the stock Google/T-Mobile ROMs are unlikely to ever add Wifi Calling support for the Nexus 5, but perhaps CM 13 or Android Marshmallow do or will have Wifi Calling support built in, so there may be some possibility of getting Wifi Calling using those.