Switching a device on and off remotely

All I want to make is a VERY simple way to allow my users to switch our equipment on and off remotely. I was hoping that this could be done with SMS messages like the good 'ol days but it seems SMS is going away (?) and coverage between areas and operators is patchy.

So, can anyone tell me if there’s a way to do this? I don’t mind writing Arduino code for a device but I don’t want my users having to download apps or browse to web pages, etc. Surely there must be a simple way to do this? (I could go out and buy cheap phones and wire up a flip flop circuit to the ringer output but that is such a horrible solution.)

Any help much appreciated!