telephone transmitter; mouthpiece
Technical term for the part of a telephone that converts sound into electrical signals; often contrasted with 受話器 (receiver).
On old telephones, the transmitter and receiver are separate.
When you speak into the mouthpiece, your voice is converted into electrical signals.
Compound of 送 (そう, 'send') + 話 (わ, 'talk') + 器 (き, 'device'). The reading is on'yomi throughout.