An ISDN (Integrated Services Digital Network) capable phone line can provide connection speeds of up to 64 Kbps (single channel) or 128 Kbps (dual channel) using a terminal adapter often referred to as an ISDN modem. The terminal adaptor removes the need to convert digital signals to analogue signals before they’re sent down the telephone line, this results in a more reliable internet connection.
The speed that data can be transferred is dependent on a number of factors:
Phone calls can still be made whilst connected.
With their purposefully built infrastructure cable companies are able to provide speeds of up to 50 Mbps - considerably faster than the highest available ADSL speed (24 Mbps).
A cable modem/router is needed for broadband internet access over cable. This is usually provided by your ISP.
The making and receiving of phone calls is not affected because the telephone line is not used.