ISDN connections are not leased lines. It's pretty much like a normal telephone connection except for the digital network which enables you to get connections in multiples of 64Kbps (i.e. BRI=basic rate interface).
BSNL's website is pretty good for finding anything related to their services. By the way, considering the current rates, it's much better to get a broadband connection, just an advice.
Anyway, here's direct link for ISDN details *www.bsnl.co.in/service/isdn.htm