Panasonic KX-T2373MXW Service Manual page 21

11.3.3. Tone Detect
This circuit is used to sense the status of the line (busy tone or dial tone) during Auto Redial.
11.3.3.1. Circuit operation
D101 → Q101 → C201 → R201 → R205 → Pin 5 of IC201 → Pin 1 of IC201 → D202 → R208 → Q201
When the subscriber hangs-up, check the intermittent tone. If cycle tone is detected, the collector of Q201 goes to a low logic.
21
KX-T2373MXW