I guess this is related to bad implementation of using SIP for connecting call. I had similar experience with local telecom provider in Turkey, which is routing international calls over cheap SIP providers. Basically they are calling a SIP route, which you hear ringing, when it starts ringing other provider starting to initiate the call.

In this case I think the cell tower is not Tmobile’s, but they have agreement with the tower owner to route call to their SIP trunk.