I thought this was primarily used to block stolen phones, or to indicate to the service provider what "features" your phone can handle. - have not heard of it being used to restrict certain phones from a network.
yes,i know that unlocked phones will work,but what about a locked usa phone,with o2 sim card in,then acticated? o2 is the official partner in the uk for iphone,so will an american phone,locked work with an apple official partner sim card now?
According to some information I've seen elsewhere there is a portion of the IMEI which determines the phones primary market. This perhaps could be used to identify US and UK devices.
Also, If I use an online IMEI analysis tool, a US iphone shows the approval body as being American and a UK Sony phone shows the approval body as the British BABT.