When you have an important connection in videoconference, and one ISDN broken, you must recall the other system to restabilish.
Sometime you loose 20 or more seconds.
If your system has the downspeed feature, as the new Tandberg equipments for instance, you still have a lower speed connection. But the computer inside will try to reconnect the broken line ?
If not, you are again forced to recall the other equipment, loosing connection time.
(Sorry for my bad english)