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Their handsets lock to the first network sim you put in.

I got stung with that. So called "sim free". The worst part is, T-Mobile are refusing to unlock it too. Absolute scum.
 
The handset will be locked to the network you get it from, say if its vodafone, o2, orange etc. You will have to request an unlocked version of the handset from carphonewarehouse. However test the handset in store with other sims to be on the safe side. If your in doubt and want to purchase a handset unlocked on all networks, you will be best going direct to Apple: http://store.apple.com/uk/browse/home/shop_iphone/family/iphone5 or get one from an Apple store.
 
The handset will be locked to the network you get it from, say if its vodafone, o2, orange etc. You will have to request an unlocked version of the handset from carphonewarehouse. However test the handset in store with other sims to be on the safe side. If your in doubt and want to purchase a handset unlocked on all networks, you will be best going direct to Apple: http://store.apple.com/uk/browse/home/shop_iphone/family/iphone5 or get one from an Apple store.

There is no such thing unfortunately. Carphone Warehouse have the same stock as the networks, and Apple themselves. The unlock is set by a flag given to the iTunes activation servers upon sale, however CPW's systems are physically incapable of giving the iTunes servers this unlock flag when they sell a phone - whether it's sold with a contract or not. This means ALL phones they sell will be locked to the first network they see.

The only phones that are truly unlocked for good are ones bought through Apple and later resold, or ones you buy from Apple yourself. The networks can unlock phones locked to their network post-sale (O2 can unlock an O2 one, for example), but they will rarely do this if you didn't buy it from them, or from a reseller with one of their contracts.
 
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