You can not make phone calls without a carrier. You could theoretically make skype calls if it has Wi-Fi.
yes
Definitely yes.
yes
The Samsung galaxy s2 unlocked
No, you need the same carrier phone or an unlocked phone.
No. You can purchase the phone unlocked without any service plan, and connect it to any compatible carrier. It doesn't matter if the carrier is prepaid or not.
You can get your cell phone unlocked through the cell phone carrier you are subscribed to. For example, if you are with Wind Wireless, there is an option on the website to unlock your phone.
It means that it can be used by anyone.
This is entirely dependent on whether you are getting a carrier subsidy or buying an unlocked handset. If you're getting a subsidized phone from your carrier, then the price is dependent on how far the carrier is willing to drop the price. If you're getting an unlocked handset, HTC and Huawei have some options that can be obtained on a tight budget. Personally, I can't recommend buying a cheap phone. The user experience is going to leave much to be desired and you will most likely end up disappointed with your purchase.
No. Each carrier has allowed cell phones for their carrier. If you have a GSM carrier though, you can buy an unlocked phone to use on that carrier even if the carrier doesn't support the phone. All you do is move your sim card.
Some phone carrier frown upon the usage of unlocked phone on their network. You run the risk of potentially voiding your contract.
An unlocked cell phone means that it can be used with any carrier. It does not have to have a contract; however, it can have a phone line on it. (any phone line, not just at&t, verizon, & sprint)