In uk.telecom John Rumm wrote:
Old voice capable modem, and a cheap low power computer like a Pi?

Doesn't even need to do voice, a bog standard modem should do. Dial the
speaking clock (020 70431320 is a geographic version from a uk.telecom
regular) or whatever, the modem negotiation will try and fail, hang up after
30 seconds regardless, job done. As long as the other end answers you've
made a 'chargeable call' even if there's no content.

Depending on your router, it might have an internal serial port you can use
to do talk to the modem if you can run a basic script on it (eg if it runs
Linux and you can get a shell somehow). Or put OpenWRT on a spare router to
do the same thing. Might need some voltage shifters for the serial port (eg
a mobile phone data cable).

Theo