These could be to a local BBS or Internet service provider. Bear in mind though, that the person you're calling must also have ISDN equipment installed to take your call. This isn't the case for regular telephone or fax calls though. So, once you have ISDN installed, there is no need for a conventional telephone connection. Most demonstrations of ISDN you'll see will probably include some form of video conferencing.
Most systems make use of both B channels.
For example, one channel could be used for a crystal-clear audio connection, and the other for a H.261 compressed video stream and application sharing. It's called H.320, and any system complying with it, will be able to interoperate with all other H.320 systems without problems. Another application of ISDN is as a leased line backup service in the business world. This is particularly evident in France, where a large percentage of the installed ISDN lines sit idle most of the year, waiting for an outage to occur on a leased line so they can kick into action. Naturally enough, the Telcos aren't over the moon with this scenario.