SDN2 lines are used for voice and data communications . They start from 2 channels (lines).
You can add more connections (in pairs) up to 8 channels . This increases the number of sconnections and the available bandwidth. It means you have built in flexibility. Your telecommunications lines can grow as the business grows. If you reach 8 channels, then it is time for ISDN 30. More than 8 channels, then ISDN30 may be better suited.
ISDN gives you access to the internet, much like a standard phone line. The key difference to a standard line is that ISDN2 channels can be used separately - so you can use the internet on one channel and receive a fax or make a call on the other.