3G isn't really anything to do with making or recieving a telephone call. It is the network that is used for access to the internet and downloading information. Your Orange SIM will work if it is a 3G one (could possibly say 3G on it in small print), if its not you can pick up a new SIM free for most networks. If you want to use the iPhone for 3G access and Wifi you will be better getting a SIM which has this for free, usually for a year or so, there aren't free but can be bought for about £30. By not using a SIM with free access to 3G and WiFI I would say you will soon go though any prepaid credit that you have.
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