Unless it has been ported away, this line is served by Level 3 Communications, a very large CLEC. It appears to be originating from Spanish Fork, UT, but as a VOIP line it may be emanating from anywhere.
While it is not unreasonable to speculate that a large company like T-Mobile could have some services with a VOIP provider like Level 3, I would not take these callers at face value. If you have a T-Mobile phone, dial 611. If not, look up the info on their web site, which I have linked for those who may need it.
T-Mobile, I called them back and the gal didn't know why anyone would be calling me. They didn't leave a message on my voicemail either. Can't figure out why they're calling me.
I accepted the call and it was supposedly t-mobile saying my account was $29 past due. When I checked online and chatted with a representative they said my account was current. I'm thinking its a scam caller.