An ISDN line can be considered as 2x “B” data channels, and one “D” signalling channel. Each of the B channels can carry one voice conversation or 64Kbps data. DrayTek ISDN routers can use 1 or 2 of the B channels for an internet connection, with the “Bandwidth on Demand” (BoD) option using the second B channel only as required.
With ISDN, if both “B” data channels are in use to give a single 128Kbps data connection – then neither incoming or outgoing phone or fax calls are possible. If both “B” channels are in use (i.e. 128Kbps data connection), Call Bumping will automatically release one of the “B” channels to be used for the phone call temporarily. After the call is finished, the Internet connection will be restored into 128K.
Sounds great because it allows you faster internet access while still allowing people to call you — except that it doesn’t always seem to work properly in Australia. Please see “What is the call bumping issue“