Man. I think you are at the same boat with me. Whenever I called and whoever I described my issue, they never heard about such thing before.
I have T Mobile Shadow and used it for 2 years. All of a sudden I started to receive calls from people saying that they are returning my calls (which I never paced). First, I didn't pay attention. Then it started to be annoying. I called T Mobile, and they replaced my SIM card. After one week of break it started again. They said that it's my phone. I upgraded the phone. Next day, - same thing. Now they are saying that probably my phone has been hacked and I have to call police to investigate possibility of ID theft. I am not sure if they are absolutely right or wrong, but I gonna do it.
Some details. People who call me have the same first 6 digits of their phone numbers as I do. For example, if my number is (123) 456 7890, the numbers of "my new friends" are (123) 456 XXXX (XXXX always different). Another detail. I talked with some people and asked them who is their provider. All of them are from T-Mobile (the same as I am). From all internet searches I found your case the closest to mine. I will keep you on track of what I am doing and asking to do the same from your side.
Good luck and keep in touch.