AT&T MERLIN LEGEND System Reference Manual page 494

Functional Description
Direct Inward Dial Trunks
Delay — The originating end of the tie trunk transmits an off-hook signal
and waits for the remote end to send an off-hook signal followed by an
on-hook signal.
Automatic — Incoming calls are routed directly to another station without a
start signal. When the user picks up the handset, the signal rings
immediately at the other end, This is also called an automatic ringdown tie
trunk.
Wink, immediate, and delay types are also called dial-repeating tie trunks.
Direct Inward dialing (DID) trunks allow incoming calls to reach specific
individuals or facilities in the system without the assistance of a system operator.
DID trunks are available only in the Hybrid/PBX modes. They are connected to
the system on an 800 DID module.
With DID service, the customer reserves blocks of DID numbers from the local
telephone company. The DID numbers should correspond to the extension
number for an individual, a calling group, or a Remote Access or pool dial-out
code.
CAUTION:
DID numbers that correspond to pool dial-out codes (or facility access
codes) can be used to evade toll restriction, leading to toll abuse and/or
fraud. (See "Customer Support Information" in the front of this book for more
information on security.)
The system can receive 1- to 4-digit extension numbers over the DID trunks. The
number of digits received on a specific DID trunk is always the same for that
trunk or for the trunks in a particular DID trunk block however, different DID
trunks (or trunk blocks) can receive different numbers of digits.
Because DID trunks allow calls to come directly to a telephone extension, they
cannot be pooled. The telephone company's central office passes the
necessary digits to the system, which delivers the call directly to the dialed
extension.
If the extension numbers used in the system are fewer than four digits but the
central office sends four, the system can be programmed to ignore the leading
digit(s). For example, if the DID number sent by the central office is 2157, the
extension numbers the system can access are 57, 157, or 2157. System
programming determines the proper extension number to connect.
The system also can be programmed to match more digits than are received
from the central office. For example, if the system is setup to match three digits
and the central office sends the number 24, the system might insert a 9 in front
of the 24 (resulting in the number 924) to complete the match and connect the
call.
Lines/Trunks 4-25