unlocked phone seems locked after erasing content

A friend of mine gave me her old iPhone (bought unlocked), and I thought I should erase the content, but when I did so, it wouldn't let me put in my SIM (which it had accepted - and worked fine - before erasing it), because it said it was locked. I did have the pin for the original SIM and put that in before removing it. Not sure what to try. Any ideas? Thanks.