This value reports how many ISDN subscribers are currently active across the device's V5 links, tracked separately from the PSTN user count since ISDN lines typically consume more timeslot capacity per subscriber. Administrators use it alongside the PSTN user count to understand the current mix of subscriber types drawing on the shared V5 trunk group's timeslot capacity. For example, a higher-than-usual ISDN user count could explain why usable timeslot capacity is being consumed faster than expected for the number of active lines.