[threedaymonk]

It seems to be more than that. For example, I typed in my mobile phone number, and it correctly identified the operator as "Hutchison 3G UK" (i.e. 3), which it now is - but it's a number that was originally Virgin and which has been ported a couple of times since then.

[alexkinch]

For mobile numbers it sounds like this uses an HLR Lookup service, which will (in simple terms) return the Mobile Country Code (MCC) and Mobile Network Code (MNC) - e.g. MCC 234 is the UK, and MNC 20 is Hutchinson 3G (aka 3). If you were to check the number itself against a numbering plan (for the UK http://www.telecom-tariffs.co.uk/codelook.htm is a useful tool) it'd show the original operator.