Never having used ISDN before I need to ask what might be a silly question: can this service be connected to using a V90 internal modem or deos it require some other sort of modem such as is required by BB connexions?
OK, sorry. ISDN usually comes in the shape of Home or Business Highway these days. If this is the case, there will be an analogue line socket and two ISDN sockets. These two ISDN sockets are in parallel, not Channels 1 and 2. Underneath is a socket to connect directly to a USB port. No modem is needed because both parts are digital.
You need an ISDN adapter (it's not, strictly speaking, a modem - but it's the ISDN equivalent of one).
However, as graham√ says, the BT Highway system has a direct connection to your USB port, as apparently the adapter is built-in to their box. The Spanish system has a similar device, actually built in to a telephone, which connects directly to a serial port.
I'm not trying to be offensive guys. The question asked was " can this service be connected to using a V90 internal modem ". The answer is no it is impossible to connect ISDN to an internal 56k or indeed any other modem. I realise that ISDN can connect to USB but that wasn't what was asked. I further realise that ISDN is dependent on the phone lines being connected up but I didn't want to confuse the issue any further, as has happened unfortunatly.