Two highlights: the call-progress tones come from the codec's hardware tone generator (it's like IQ had this in mind?) and the software is multiprocess behind a board HAL (including a mock backend so the phone logic is unit-testable without the hardware). Repo has the PCB/BOM/parts too, plus a 3-part write-up. Happy to answer anything.

Write-up: http://blog.1in1e6.com/2026/06/payphone-part-1-new-old-phone...

(for the AI crowd - I architected it, but used Claude Code to fill it in; that's part 3 of the write-up.)