Is the phone locked to ONE simcard, or to a specific phone company? eg if I have to simcards from the same company, providing the iphone, will I be able to swap between them?
You need to unlock the iPhone for the same network.... (pay your carrier to allow you to use any plan of your wish on your iPhone) - any sim from the same carrier will work!
Unlocks (pawnage types) and official free carrier (~$1500) will get you any carrier, any sim!