I have an iPhone 3G with 2.2 firmware, and it was working fine since i had bought it, i never removed my sim card from it since i had bought it, today i removed my sim card to put it in another phone then i put it back to my iphone but now it tells me there is no service and i can't make or receive any calls and when i remove SIM card while the phone is Powered on, it tells me "no SIM card inserted" and when i put it back still no service.
You will have to contact a Mobinil or a Vodafone, whichever you bought a phone from and request a new SIM card. The SIM card that comer with the iPhone is not provisioned to use on several phones (normally, in legally activated phones).
the best thing that you need to do is call your local service provider's Customer Service. ask them if the SIM CARD is provisioned in their SYSTEM (since i work in a telco and this is how we do things).