I am from India, and this was produced back in the 80s by a company called ITI.

 

You need an adaptor
https://www.broadbandbuyer.com/store/voip-adapters/
but most of them require telephones which have DTMF dialling. One with a rotary dial will probably use loop-disconnect.

 

Nice idea. I'm pretty sure there must be pulse to DTMF converters too, but how about using an ESP32 processor to do the whole job. https://github.com/sikorapatryk/sip-call