How can an AT&T iPhone 4S - be unlocked and made to work with Sprint?

I have an iPhone 4S on the AT&T network. Since the iPhone 4S utilizes GSM and CDMA networks is it possible for the phone to be unlocked and then used on the Sprint network with a registered Sprint phone number?

Not possible.
att is gsm and sprint is cdma. There's no way to make it work.
sell your 4s and buy a sprint.
if your in PA i have a sprint iphone 4s, its new but i'm selling it to get a att iphone

It is not possible.

So my sister unlock iphone who lives with my parents got approved unlock iphone for a mortgage loan but not the amount. They need so my family unlock iphone asked me to cosign the mortgage. They live in CA unlock iphone and I live the way in NC. I was planning unlock iphone to buy a house here. But now I can't unlock iphone because of this issue.

I'm sorry to say this but it can't be done.

Why?

ATT uses a SIMcard like kind of tech which means you will need to transfer to other carriers to use the same thing. I know T-Mobile also does the same kind of tech so you can do it with them.

Sprint uses something else that does not require sim cards so the two things will not match.

Good Luck, also if you do need unlocking I would recommend this website: http://iPhoneunlockshop.com