Nortel DMS-100 Series General Description Manual page 131

Ringing system maintenance 5-7
In addition to the DMS-100 MAP terminal, feature package NTX215 allows a
DMS-100 to be monitored by a No. 2 SES. This feature can be used to monitor
calls through the DMS-100 to isolate intermittent ringing problems.
DMS-100 Family DMS-100 Ringing System BASE15 and up