After some 'hummm, how might that be done' type thinking, it would very much depend on how and where the phone line was terminated at within the old analogue ph

After some 'hummm, how might that be done' type thinking, it would very much depend on how and where the phone line was terminated at within the old analogue phone. In most of the ones I have dissembled the cord is pretty much run directly to the handset hang-up switch. If that's the case then I doubt it however a few that I've seen have terminated the cord on the main circuit board with some downstream components for various functions (call/line indication, remote ringing etc) before running up to the on-hook switch. If the phone is one of those ..... .. Then yes, I suppose in a James-Bondy way it might in theory be possible (with sufficient gain, filtering and DSP) to (ab)use an electrolytic capacitor, piezo buzzer (hard-of-hearing ringing booster) etc. If it is one of the simpler ones, then no (but not a domain expert). Best suggestion is open it up and see how/where the cord terminates. If there is little between it and the cord coming in then I don't think there is any way to 'jump over' the air-gap created by the switch though I guess you might be able to measure the change in air capacitance of the switch air gap for very loud noises (spinning rust disks record loud vibrations/sounds) so as always, consult an expert based on threat model. In terms of legal/type approval any testing most likely states that ‘phone must disconnect from line’ and be able to withstand x amount of surge before emitting magic smoke. Local laws may vary. Note:VOIP phones are a different kettle of fish and haven't taken enough of them apart to comment on them.