iPhone loses WiFi 3G signal wont accept S code and locks out

Ive had this happen twice this week.
Ill be using my iPhone and suddenly the Wi-Fi icon disappears. A split second later the carrier vanishes and the bars disappear leaving only searching where the signal used to be. When I try to use the phone it says my SIM is locked, and I need to unlock it. However, when I type in my SIM unlock code, it doesnt accept it. It tells me there are 2 more tries left, but then goes straight to PUK code entry screen without letting me attempt twice more.
I know my SIM code is correct, because when this happened the first time I reset it and wrote it down. When I used the code the iPhone wouldnt accept it.
So twice Ive had the hassle of contacting my phone provider to get my PUK code.
But why is this happening? Has anyone else had this happen?
iPhone 3GS 16GB iPhone OS 3.1.3 Model MC131B