yes, it's t-mobile.  Why they don't leave a message is stupid.

I think it's a local T-Mobile number. If you are late in your payment, that's who it is.

Calls irregularly, but with some frequency. Never leaves a message. Just don't answer ANY call if the number is unknown.

phone rings all the time.. i do not answer.. leaves no message. annoying

This number has called me twice today, once extremely early, but no message is left, and I'm getting very annoyed.  I haven't answered it yet.