Nortel 1000 Configuration Manual page 217

ZDP
Zone Digit Prefix. This is the number that the main office appends to a local
SRG PSTN call dialed from an SRG IP Phone in Normal Mode. This number
differentiates the call from a main office PSTN call dialed by the main office
telephones. The ZDP routes the call through VoIP trunk to the SRG.
Copyright © 2005-2007, Nortel Networks
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Nortel Communication Server 1000
Main Office Configuration Guide for SRG 50
NN43001-307 02.02 Standard
Release 5.0 3 December 2007
List of terms 217