I just got an iPhone 4 passed down to me. A few questions?

I had an iPhone 4 passed down to me. I read online that you can not use prepaid phone cards on the iPhone 4. Is this true? Also, I'm pretty sure the SIM card is internal on the 4(verify this?), so does this mean that I will have the same number as the person who passed the phone down to me? And is there any way for me to have my old number off of my old phone if I use a prepaid card? I have Verizon, but I don't plan on activating the phone with Verizon, as I want it prepaid.