How do GSM+SIM cards work? Do I need a service provider or do I just stick in the SIM card and call?

Ok lets say I buy an unlocked GSM phone in the US and a prepaid sim card. Can I call people w/o a service provider or do I need a service provider? If I do, is the service provider free or are there monthly bills in addition to the sim cards?

How do GSM+SIM cards work? Do I need a service provider or do I just stick in the SIM card and call?
they are service provider specific and in most cases it will lock you out unless the sim from the same company... you really need to be sure the phone will support that type of card.
Reply:If you bought a pre-paid sim card, you should have some sort of idea who your service provider is????? You can't just buy a pre-paid sim without a provider. T-Mobile is the largest pre-paid sim card service provider on the GSM network. There is a good website about phone service that has really good links including those about cell phone service and it's worth reading. I'll include it below. They are continously upgrading and adding relevant information and they have an RSS feed for that too. There is a really good link that pertains to nothing but service providers and examples of coverage and I can see why they added the link. Their info is valuable too and worth reading and bookmarking!