It reminded me of alarm central station protocols. The earliest (e.g. 3/1 and 4/2) use pulse, and the later ones (Contact ID, SIA) use DTMF [0]. These systems a

It reminded me of alarm central station protocols. The earliest (e.g. 3/1 and 4/2) use pulse, and the later ones (Contact ID, SIA) use DTMF [0]. These systems are still installed and relied upon by alarms today. It does seem the industry is finally coming around to LTE, since a copper pair is basically a fiction we present to alarms at this point. [0] https://www.emergency24.com/cp/formats.aspx https://www.emergency24.com/cp/formats.aspx has a cursory overview of many protocols