RJ9 Connector

Telephone handset cords (coiled or straight) almost universally use 4P4C connectors on both ends.

The inner pair (pins 2 and 3) carry the microphone (TX) and speaker (RX) voice signal.

The outer pair (pins 1 and 4) are used for chassis ground or wiring inversion and usually carry no signal, as symmetrical wiring helps prevent polarity issues when reversing cord orientation