en-digit telephone number. The first three digits of everyone's number are the same. The telephone numbers that work with DID trunks are purchased from a local telephone company in blocks, usually of 20 or 100. To the caller, these are indistinguishable from regular telephone numbers. Large companies using DID may assign the first seven digit DID number of the group as their main telephone number and have that number answered by the switchboard. The purpose of DID is that you do not need a separate trunk for each telephone number. You may have 10 DID trunks with 100 DID directly dialable telephone numbers. The as- sumption is that no more than 10 telephone numbers will be called at any given time. If an eleventh call comes in, the caller hears a busy signal. When a DID telephone number is dialed, the central office recognizes the number and sends it to the PBX. Once it reaches 210 8 The Telecom Handbook Figure 10.2 Direct Inward Dial Trunks (DID) DID Trunks PBX Directs Incoming Call to the DID Extension Number Combination Trunks Used for Outgoing Calls Only. 212-883-1242 (DID - Last 3 or 4 (1242 is the DID Extension Digits Sent to PBX Local Telephone Co. Number) Central Office Local Telephone OUTSIDE LINES the PBX, the last 3 or 4 digits (important to specify which when ordering - sometimes known as 3-digit outpulsing or 4-digit outpulsing) are repeated to the PBX. This instructs the PBX as to which telephone the call is to be directed. DID trunks in the U.S. are wink start trunks. The wink is a signal that the PBX sends to the central office to let it know that the PBX is ready to receive the call and the digits dialed. In Eu- rope, DID trunks are immediate start, meaning that there is no wink and the digits are sent immediately. Once a call has reached the DID extension, the PBX treats it just like any other extension. When selecting a DID extension on the telephone instrument and dialing 9, you are not dialing out on a DID trunk. You are dialing out on one of the combination trunks with which your system is al