I can't get my iPhone 3GS to work? SIM card problem?

I recently purchased an iPhone 3G. However, when I put a SIM card into it and try to set it up, it displays this message:

The SIM card inserted in this iPhone does not appear to be supported.
The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information.

The phone has just been stuck on that screen and it displays 'no service' in the corner. I've restored it several times and it still isn't making any difference. I really have no idea what I'm meant to be doing, am I supposed to jailbreak the phone?

Any help would be more than appreciated.

I got my iphone unlocked on unlockiphoneforfree.tk. Try it might work for you.