SIM card for cell phone question.

You are correct that an unlocked phone is needed but it still doesn't mean it will work automatically. I sold an Iphone 2 years ago that was unlocked and had been tested with 3 different sim cards for US and Canada and was working fine but would not recognize the Mexican sim card my buyer was trying to use. After researching online, it was a known issue with that particular network and there was a workaround for it but it wasn't easy for the average user. Do your research before bringing your phone...