Not sure if this is the best place for it - perhaps PM a mod and see where they think the best place would be.
As far as I know, the ISDN lines are used to carry more than 1 phoneline, and as they are digital, at a better (or comparable) quality. By the sounds of it you are using a Digital Hybrid system (parents office uses a similar Panasonic system).
In all honesty, your best bet is to phone your telecoms provider, or if not, the person/company that installed it, and ask them what the best options for change are
Hope this helps,
Dave