any unlocked phone + any unlocked sim = able to get prepaid service?

Hello, I have been searching the internet high and low for this answer and found nothing...mostly because I feel its kind of a hard subject to search on.....too many possiblities. Anyway, my friend is England has told me you can buy any unlocked phone (which i got off ebay) and an unlocked SIM card (also ebay, an AT&T one) and get prepaid service. I do belive him that it works in England, but i was curious if it works in the US? I tried to activate this sim card but they gave me the run-around (always seemed to ask for my wireless phone number, which i dont have yet), and in the end they told me to goto my local phone shop to get a number assigned to me, which i will do later. I was curious if i need to get a special SIM? I dont mind having to buy a new sim card since they are cheap, but if the idea doesnt work at all i will be out $150 alone just for the phone! Any ideas??

if you getting a phone and want to work international, make sure that this is a tri-band phone.900mhz/1800mhz/1900/ 900mhz work in asia 1800mhz work in europe 1900mhz work in u.s and canada.
if you getting a at&t sim card it's gonna work in uk. this is the list of all uk service provider. 02 cellnet, guernsey telecom, hutchinson 3g, jersey telecom, manx telecom pronto, orange, and tesco mobile. unless if you have a plan and activate the international roaming which you have access all over the world. exept japan and korea. im just curious! what is unlock sim card? never heard it before what i know is unlock cellphone but not unlock sim card. hope this will help you.

you can get a t-mobile pre-activated sim card at ebay i think $20. once you recieve the sim card it is already activated and ready to use but the area code from diff state. just call t-mobile customer service to change the area code to your local area code. t-mobile will now asking for some personal question like name, addrs, d.o.b some personal question and that phone number that t-mobila give to you will be under your name.

Sorry, by "unlocked sim" i meant basically a prepaid sim...one that doesn't come with a phone thats under a regular plan. Anyway, I went to the local shop today but it was too late! They closed at 3 and i got there at like...4:30. I will now have to wait until monday to get my sim activated and a phone number, oh well! Also, i am 99% sure this will work as i emailed the guy i bought the sim from and he told me the same thing Cingular/AT&T cust. support told me - gotta goto your local shop! Thanks for all the help!