Return calls from people I never called, Samsung Blackjack

I get at least 3-4 calls a week from people saying I called their cell and did not leave a message. They are returning my call based on their phones call log. I am not butt dialing or in any way making these calls. These callers are not known to me in any way and are not in my contacts list. They are from all over the country. My phone sits on my desk all day and never moves.
Does anyone know what is happening?
I have a Samsung Blackjack with AT&T service and this has been going on for months.

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.