There was an article in todays Guardian titled “London calling?” and the growth of people adopting London telephone numbers for their non london businesses via VOIP services (skypeIN for example).
The article basically says “hey look isn’t it cool that these people decieve other businesses into believing they are more established than they are”.
You could check a business’ telephone number against the ranges that these providers use to see if it’s virtual or not but that’s too much like hard work.
The underlying issue is that the telco’s national call revenue will get eroded faster than local traffic. If we all adopt virtual London numbers then there would only be local calls and zero national. We know that (and they’ve admitted) that their revenue is declining. But this will speed up the decline in national rate calls faster than it already is.