so equipped. DID trunks are for incoming calls only. A DID trunk is a switched circuit. If DID trunks are delivered on a PRI type of -1circuit they may also be used for outgoing calls. See more on this below un- der PRI T-1's. The delivery of the DID digits to the PBX can be analog, where the actual digits are sent using audible touchtone signals or digital, where the digits are delivered using combinations of 1's and O's to represent the dialed digits in binary code. Tie Lines A tie line is a point-to-point line between two PBXs. You may occasionally hear the term dial repeating tie line or E&M tie line (stands for Earth & Magneto, Ear & Mouth or Emigrate & Migrate, depending on who you ask), which is the same thing as dial repeating. The idea behind a tie line is that you can dial the extension of someone on a PBX at a distant location without placing an outside call through the public network and without talking to the switchboard attendant at the distant PBX. A tie line may be accessed by dialing one or two digits into your PBX; for example dial 8. Once you dial 8, you will hear dial tone (also 212 - The Telecom Handbook Figure 10.3 Tie Lines Any Telephone in the NYC Office Can Place a Local Call in St. Louis Using the Tie Line Tie Line EXT. 7422 Tie Line Local Telephone Co. Central Office (St. Louis Local Call) NEW YORK ST. LOUIS EXT. 7421 Any Telephone Can Reach Extension 5812 in EXT. 5812 St. Louis Over the Tie Line by Can Dial Ext. 7421 Dialing 5812 (or Any Tele- in New York phone Extension on the St. Over the Tie Line Louis PBX) OUTSIDE LINES called drawing dial tone) from the distant PBX, you may then dial any extension within that PBX. If the distant system is pro- grammed to allow it, you may also be able to dial 9 instead of dialing an extension number. This will enable you to place an outgoing call through the distant PBX. For example, if the tie line connects Seattle to Los Angeles, the person in Seattle may dial through the Los Angeles PBX using the tie line and be charged only for a local call in