Very detailed video on Russian phones with built-in CallerID systems, which usually had features far beyond number identification (metronome, guitar tuner, alarm clock, music composer, voice changing).

The video is interesting not only from the historical, but also from technological and programming point of view.

https://youtu.be/DvfgnR6mVDI?si=VDRPSzXKnSA1bZQQ&t=1402 (23:21) — "Rus-28 Sonata" music on a Spectrum's YM2149 chip

https://youtu.be/DvfgnR6mVDI?si=W1IQGhcyxUnyn8lK&t=1574 (26:14) — HQ polyphony using 8-bit output of Intel 8052 clone.

https://youtu.be/DvfgnR6mVDI?si=X8uRz3geBbcxWj0-&t=3050 (50:50 - 01:00:17) — how this polyphony was implemented

https://youtu.be/DvfgnR6mVDI?si=0hrcI-P75zHXQt98&t=2775 (46:15) — poor man's FFT on 1-bit ADC using comparator, for number identification and guitar tuning

As one of the comments says, "Once upon a time, the Rus'-based CallerID phone was the only form of entertainment at home..."