ISDN is the next logical step for connectivity of sites over the wide area that need more bandwidth and reliability than POTS connections, but don't need the volume of bandwidth and cost of very high-speed dedicated links such as frame relay or T1.
Nothing comes for free.
Most of the solutions to our problems that appear too g ood to be true either are too good to be true or they come at tremendous sacrifices. In the case of networks, the solutions usually come at the sacrifice of funding, flexibility and reliability of the technology that we are implementing. Although it has taken ISDN years to evolve into a reality, ISDN is now popular for being one of the most uncompromising solutions to wide area connectivity and remote access, and it will hold that status for some time. ISDN, however, is not too good to be true. It is not the easiest technology to obtain, implement or manage.