I recently purchased an unlocked iPhone from the Marketplace, and I had a question to ask in regards to testing it out. The seller had the iPhone unlocked by AT&T. Now, without a Sim in the phone, the iPhone asks (when connected to iTunes or not) to "Insert a Valid Sim card". I wanted to go through the phone and test it all out, but all I have are AT&T and Straight Talk Sims. My family member that's getting it wants to use it on T-Mobile. Can I activate it with one of my Sims to start, or will that lock it to a carrier and prevent her from using her T-Mobile Sim when she gets it?