ISDN is a digital service and currently incompatible with ADSL broadband. To get ADSL broadband you would need to convert your ISDN line back to a standard analogue telephone line first.

Remember if your ISDN lines were installed to provide Internet access and you have other normal telephone lines - why not put the ADSL service on an existing 'analogue' line (fax machine etc.?). You may not need the ISDN lines at all saving you around £80 per quarter in line rental?