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.