Can I activate my old iPhone for Verizon even though it was originally an AT&T phone?

Can I activate my old iPhone for Verizon even though it was originally an AT&T phone?

Added (1). I bought the phone for my att plan but I would like to give it to my mom who has Verizon. How do I set it up? She doesn't have a data plan but would like to be able to use it when connected to Wi-Fi and make calls normally

You'll need to get AT&T to unlock the phone, BUT, I don't know if it will work on Verizon as Verizon uses CDMA, while AT&T uses GSM.

Just so you know, all smart phones pretty much have to have a data plan; it's not optional.