ISDN lines are digital phone lines that used to be used widely for VC, with numbers that looked like phone numbers.
Typically 6 lines were bonded together to give a bandwidth of 384 Kb/s. This is slow by modern standards, and the line rental and call charges very expensive, so ISDN has now been almost entirely replaced by IP (internet) connections.
ISDN lines (acting as standard phone lines) are still used to provide telephone access to our bridges and VC units, as well as external sites who still use ISDN.