Cannot make/receive calls, but data and SMS work. Help.

I have a Oneplus One on the Tmobile US network. I have no problems with data and SMS but sending and receiving calls is problem. My mother has a Oneplus One also on Tmobile that can send and receive calls from anywhere. However mine can only send and receive calls in one spot in out backyard. I have done the following without success.

1) Turn off/on phone, switch from 4g to 3g to 2g, toggle airplane mode

2) Switch sim cards between phones. Her phone still works great with my sim, and my phone is still terrible with her sim.

3) Verifying that IMEI is present (via *#*#info#*#*)

4) Verifying APN.

5) Factory reset to match the OS and firmware as her phone.

6) Trying different firmware and OS/custom ROM.

I am not sure what else to try.

I am beginning to suspect that the antenna is failing. I suspect I am likely going to have to buy a new phone, but I wanted to see if there was an easy solution. I really liked this phone, and want to find a away to save it.