As someone mentioned, if it is in digital mode, it is nearly impossible. HOWEVER, it is possible if you're allowing your phone to drop into analog mode. I made this mistake when I was a cellphone no0b. About a month and a half after getting my first cell phone, Sprint called me to tell me that they were cutting me off. "WTF?" I thought, and it was really bad timing, believe me. Anyhow they figured out that someone had "stolen my number", and that it probably happened because I hadn't configured my phone to be digital only.
In the end I had to change my number, and erase my no0bishness by changing a simple option on my phone. So, check your phone!