How could I carrier unlock an att iphone 4s to use it on verizon?

My friend gave me an old iphone and I want to use it by its att and i'm on verizon. How, if at all, would I be able to carrier unlock it?

Have to buy an unlock code. ATT uses GSM and Verizon uses CDMA signals. They are not cheap either. Best luck.

You can't use it on Verizon, unlocked or not. Period. GSM phones don't work on CDMA networks, not a thing you can do that will make it work.