Bothersome calls from cellphone numbers

A friend lives in the Detroit area and is receiving a ton of calls from
numbers that show up on his Caller ID as being from the 313 area code
and the 415 or 445 exchange. The numbers after the 415 or 455 are
always different. Strange messages have been left on his answering
machine, but so far he has never talked to a live person who can tell
him why they're calling. One message did say something about his
number being programmed on their T-Mobile phone. Does anyone have any
idea about what could be happening? Are numbers pre-programmed on
T-Mobile phones?