@ Andrew Rodland
Sorry Andrew..... not correct.
Just becaus it takes a sim card does not mean it is 'approved'. All the [UK] networks have approived and non approved devices. There are some good and bad reasons for this, one good one being that some cheap chinese devices do suffer from radio leaking, ie, skipping around the 900mhz areas and interferring with other devices and infact other frequencies.
Another [good ?] reason is the provision of service. The reason you are able to put a sim card in a device and have all the correct settings 'sent' to you is that the NOC and the device 'talk' to each other, the imei contain information about the type of device it is. This in turns talks to a Auto' Device Config' tool the NOC buys form the supplier [Ericsson / Nokia / Nortel et al] that is linked to a database of 'approved' devices.
It is perfectly possible to have a 'chinese knock off' device, some sort of 'nokia me too' clone running on a network, but it is also 110% possible for that same NOC to block that device [and subscriber] by blocking the imei. I have three devices and one of them is , in fact a chinese knock of nokia thing which I bought in Africa.
In the UK it 'feels' like we have 'open access' but we don't. We just have a massive choice of 'approved' devices that allow users to skip and hop around the NOC with a massive range of devices, which is why you can take an old Nokia [1611 for example, that uses the 'big' sim format ] but a £5.00 per paid sim and make a call and send an sms. Obviously no 2.5G or 3G, but what da ya wan' for a fiver.... ?