Most LEC's charge per-minute for ISDN. Pac Bell and SWBT are some of the few exceptions. What's even worse is in some states there isn't even a residential ISDN tariff, hence all ISDN lines are "business" lines and billed accordingly. The only real solution is to demand dial the connection from both ends. This is fairly straightforward assuming your provider is set up for it, since there is a lot of ISDN equipment that will brfing the connection up only when there are packets to send then idle it out.
Here in SWB territory there are residential and business rates for ISDN.
An economical alternative in some areas is frame-relay. In Washington state USwest wants ~$70/month for a 56k FR link to anywhere in your LATA. The providers around here charge $100-$150 to link a single machine to the net via FR, and $150-$350 if you are routing a whole subnet.
Be shure to multiply these costs by 2 since a singe 56k is approx. half of 2 B's that have been bonded. I specificaly looked for a provider who didn't charge according to how many machines I have on the other end. I am buying the ability to send a certain amount of bits over a wire in a certain amount of time. Whether those bits come from 1 or 100 machines is irrelevant. I would have no problem w/ interNIC chargeing for registering IP's (which I hear may happen sooner than we realize) since I got a full 'C' when I set up. We already use 23 of the addresses and will be looking at adding at least a couple of more in the near future. In the case of the CombiNet 160 the choice of 'idle-out' or 'tear-down' is one of software. The codec is programmed via a serial line (which the dox say can be hooked to a modem, as if anyone was that stupid) port and has its own little menu system. I keep both my B's running all the time. Take care.