Do you ever look at the phones on ebay, that say unlocked to all networks except 3? And wonder why that phone you want to buy, says "not on the 3 network".
Well, when a seller states that a phone is unlocked to all networks, it is actually unlocked to the 3 network as well. Your sim card would work for a few days, but then you will get a warning from 3 and if you ignore that, then they will disconnect you. So it is not the unlocking of the phone, it is the fact that 3 will not let you use the phone.
Why? Well, here is the simple non technical version. There are three types of phone, 1G, which was the big clumpy analogue sets of the Nineties. Then we went digital and they were 2G and now we're moving to 3G. For simplicity (but not technically accurate) think video mobile phones. Now there are still a lot more 2G phones out there than there are 3G.
The "3" network only operates on the 3G (think video phone) system, but the coverage is still mainly in cities and large towns. So what happens when you're not in the coverage area? They use Orange as a back up (used to be O2) and for this they have to pay, a sort of roaming cost. So, if you're not in their coverage area they make no money. Now, if you put your sim card in a 2G phone that has been unlocked, it will be stuck on Orange as it can't pick up the 3G signal. Which means no revenue for "3".
So they will send you a message telling you to revert to your original phone. How do they know, you're using a different phone? Two ways, one your phone relays info about it and secondly your phone tells your provider where you are. So if you're in a 3G area but your phone is stuck on Orange instead of switching back to "3", then they will also know.
So, are you stuck with the handset you got with your sim? No, if you purchase a 3G phone which is unlocked then you can use that. So, just to clarify, 3G is a phone type, "3" is the network name of the provider just like O2 or Vodafone.