Can I keep my iPhone and number if I switch from AT&T to T-Mobile?

I have an iPhone 4S running iOS 6.0 and I bought it around May or June of 2012. I have had the carrier AT&T the whole time I have had this phone, but my parents would like to switch our service to T-Mobile. Everyone in my household have upgraded to newer phones, but I have kept this phone the whole time.

I heard T-Mobile makes you buy a new phone if you switch to them. Is that correct and how would it apply in this situation?

Am I able to keep the same phone number?

Am I required to use an iTunes backup, and can I do that without having to upgrade to iOS 7?

Why the hell r you going to switch to t-mobile?

You will most likely be able to keep your own number when switching to T-mobile, and you can keep your phone if you get it unlocked by ATT and then switch the SIM card to a T-mobile sim card. You will be able to use a pay-as-you-go or no contract plan when you own your own phone, which is cheaper than a contracted plan.

It is a pain in the *** to have ATT unlock your iphone. First you will have to make sure that your contract is up, or else they will not unlock the phone. Then you will have to contact customer service and have them unlock your phone, they will be unhelpful and it will take a while for this to get done. When I had my iphone unlocked ATT sent me to Apple, and Apple can't unlock the phone because it is owned by ATT. You also can't have your iphone unlocked in a store.

I'm not sure about the Itunes backup, but I think that if you upgrade to IOS7 your phone might run a bit slower.

You would have to ask AT&T for an unlock request on that phone. AT&T are the only people that can properly do it. Then once the request has been sent wait 24 hours and restore the phone through iTunes on a computer. Back it up first of course. This will also update the phone to IOS7. Now, if you want to keep IOS 6 then you can't unlock the phone; restoring MUST be done to apply that change.