Most of the law on misleading advertising applies only where adverts are aimed at consumers, rather than at businesses.

However, resellers/providers could potentially be challenged in a number of ways:

-if a service provider (i.e. the company which invites consumers to contact them on a NGN) makes false or misleading statements about the cost of calling, and if they made those statements because of false or misleading information provided by the reseller/communications provider, then the reseller/communications provider could be liable under the Consumer Protection Act 1987 (report to Trading Standards / Consumer Direct)
-if a communications provider makes false/misleading statements about matters other than price, they might be caught by the Trade Descriptions Act 1968.  For example, if they said that routing services were available exclusively on NGN's and if it could be shown that this was not the case.  Report to Trading Standards / Consumer Direct.
-if a communications provider knowingly misleads a service provider into taking up an NGN, this could be theft.  Report to Trading Standards / Consumer Direct.
-if a communications provider's advert is misleading, this could possibly be considered by the Advertising Standards Authority.  Adverts do not include the company's own website, but banner adverts (like the ones which appear on this site!) may be covered.  Report to the ASA.
-if a service provider has been misled into taking up NGN services, they may be entitled to cancel their contract with the reseller/comms provider and/or to claim compensation for any losses they incur as a result.  This would be for the service provider to consider themselves.

I'm not sure how far OFCOM would or could go in enforcing the letter and spirit of the numbering plan against comms providers or resellers.  Might be worth asking them.