Nortel ICS Install Manual page 107

Tips -
provide DTMF receivers for every two auto-answer loop start
trunks on Loop Start or CI Trunk Cartridges.
DID and PRI trunks cannot be configured to answer with
DISA. If you want incoming DID or PRI calls to be answered
with DISA, configure the system with a DISA DN. Incoming
calls that map onto the DISA DN are then routed to a line that
has DISA.
In addition to public network connections, Norstar Modular
ICS can be integrated into an existing private network, or to
other Norstar or Meridian 1 systems to form a corporate
telecommunications network.
N0130943 02
You must have one E&M/DISA Trunk Cartridge to
Controlling system access / 107
Modular ICS 7.1 Installer Guide