thanks for the infomation...
In the UK, iphones were locked by the network so they could only work on o2 (to start with) and to unlock them to work on another network (Such as Orange/Vodafone etc) would have invalidated their warranty. I.e. Apple won't fix it if it breaks
If you still have the instruction leaflet, check to see that this is the case. If your prepay sim is on the same network as your contract then it should be fine.sidekick or iPhone??
Is your iPhone unlocked from network restriction?
If yes, You can use any GSM SIM card with your iPhone no matter if it is a prepaid SIM.
If No, You can use a same network's prepaid SIM with your iPhone. If you need to use any other network's GSM prepaid SIM, You need to unlock your iPhone. You can unlock your iPhone safer and easier using remote unlocking service. This method of unlocking is safe, simple, affordable, quick and permanent. It doesn't require a jailbreak too. You can get the remote unlocking service for an iPhone from