Go into Settings --> Phone --> Sim pin and enter your pin code to unlock.
If you don't know the pin the only way to unlock it is to call the company who sold you the SIM and get the unlock codes from them.
Thanks! I'll take it into the store. I just got this phone about 3 wks ago and this is the second time this has happened. I think that both times it was after powering down, which I rarely do. In your opinion, do you think that there is a problem with my phone?
Have you disabled sim lock and then it spontaneously comes back on? I have an iphone and I have sim lock disabled and I just use the phone lock instead.
If it keeps turning back on without you doing it then you might have a problem with your phone.
I wanted to follow up and tell you that I ended up calling AT&T to see if they could do it over the phone and they were able to. Basically, they gave me a bunch of numbers to try and then my phone said I had run out of attempts and to enter the PUK code, which they had. Then it prompted me to enter a new PIN. I did. Now I've gone in and turned off the PIN feature as you suggested. I've locked my phone with the PIN, not the SIM.
Thanks for your help!