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Ear and Mouth, or E&M, is a type of trunk used by the phone company, to connect to PBX systems, or to connect two PBXs together. It is designed for answer supervision, in other words, each end knows exactly when the other has hung up. It is designed to be very reliable.

The name comes from the fact that each end has a connection name "Ear" and one named "Mouth." These connections are for getting and receiving signalling, respectively. When connecting the line between devices, the "ear" from one device connects the "mouth" on the other.

To initiate a call, the end making the call grounds its "mouth" line. The other end detects the voltage change, and signals that it is ready by grounding its "mouth" line. When the call is disconnected, the "mouth" line is disconnected from ground.

The voice is carried over a second pair of wires, and in some types of E&M, one pair is used for carrying voice in each direction.

With the move to digital trunking, E&M signalling continues to be used, but the Ear and Mouth signalling lines have been replaced with a single data bit which is set to 0 or 1. See Robbed Bit Signalling for more information.

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