Can you use an old iPhone 3gs if you brake your new iPhone 6?

One of the four iPhone's 6 in a contract, not yet expired, was dropped and broken beyond repair. I have an old 3gs that is dead or, no service, as an expired phone. Can I use this and reactivate this phone till the contract expires? I'm not buying another phone. I do not like smart phone and I'm going back to a flip up when this contract expires.

If you are making payments for the phone, you'll have to pay for it still. If not, just pop your sim card in the old phone and boom. You're done.

I understand that I will have to finish paying for this but it is really that simple?