ABSTRACT
A simple bi-directional signaling method developed as a standard interface connecting a phone switching system (such as a PBX) to an interoffice trunk circuit. Normally, there is a single phone number associated with a “standard” ana log phone line, such as the one in your home. With Direct Inward Dial (DID) and “Ear” (or Earth) and “Mouth” (or Magnet) or E&M circuits, more than one number may be associated with one line. DID trunks usually connect cus tomers with phone companies while E&M circuits can be used to provide internal connections between PBXs. Both DID and E&M type trunks provide similar functions but with different technology. With both DID and E&M circuits, the dialed phone number is prefixed to the call so the equipment can deter mine the number called.
