Can I buy a prepaid sim card and just use it in any cell phone?

My friend is coming to visit me for a month and I am planning to buy a Cingular prepaid sim card off ebay for her. Can I just buy a sim card, simply insert it in the cell phone, and begin using it? or do I need to registered with the company or join any plan?Can I buy a prepaid sim card and just use it in any cell phone?
A few concerns you'll need to make sure of with doing this, in your situation:



A) The phone has to be either Cingular/at%26amp;t OR unlocked, otherwise the SIM will be rejected out of hand.



B) Make sure the SIM has NEVER been activated; if it has, it's likely to have someone else's info on it, if it's still active; if it's been cancelled, you just bought a pretty SIM that no one can do a thing with. This is a big concern especially with buying off of eBay.



C) Once you have the SIM and a compatible phone, you WILL need to contact ATT to set up a phone number and minutes structure, activate the SIM, and put some money on the line.Can I buy a prepaid sim card and just use it in any cell phone?
yeah. of course you can do that. just make sure that you are using a cell phone that's openline so that you can use any kind of sim. you don't have to register to a carrier.Can I buy a prepaid sim card and just use it in any cell phone?
Yes you can