I don't really know a hell of a lot about ISDN, as I've only ever been a home user with a 'modest' budget. However, I seem to recall something called dual-channel ISDN. That might explain the doubled bandwidth figure, or it could just be me making things up. Who knows?
On a serious point, is there no broadband supplier in your area, or is it an objective decision to use ISDN? Just curious.