I need to put a Verizon SIM card into a iPhone setup with AT&T and its not letting me

I need to put a Verizon SIM card into a iPhone setup with AT&T and its not letting me - 1

Your iPhone has to be unlocked in order for it to work. Though, even with an unlocked, phone, it's not guaranteed to work.

1, is the iPhone unlocked so that you can use it on another carrier?

2, Does this iPhone also have a domestic GSM radio in it, so that it will work on a GSM networked phone in the USA?

3, If this is a CDMA only phone, it is only going to work on a carrier with a CDMA network (Sprint and Verizon are CDMA, T-Mobile and AT&T are GSM).

You can't unless you unlock the phone (google it) because it is probatbly locked to at&t.

Verizon (CDMA) and AT&T (GSM) use incompatible network types. It's likely that your phone doesn't support CDMA protocols and frequencies.

Obviously. Verizon is CDMA. AT&T isn't.