Those old analog tone systems were a beatch. Back in the late '70s and early '80s, the SCADA RTUs and MTUs (Supervisory Control and Data Acquisition Remote and Master Telemetry Units) that I designed and maintained for Amoco Oil Company used 600 baud analog modems. For pipeline applications, I'd have remotes over a 1000 miles from the master. The telephone lines connecting them passed through several small, rural phone companies. The lines had tone-operated loopbacks for troubleshooting purposes. Every once in awhile, the analog components in a modem would drift into the loopback tone range. Instant dead end, black hole for comms. And that usually happened on a Friday evening after workers at those rural telcos had left for the weekend. Pipeline pump stations running all weekend with zero control. And that's when pipe ruptures or dig ups tended to happen. Ah, the joy.
Once digital components for modems became available, I designed a piggyback card that could be retrofitted to existing remotes with some trace cuts and soldering. I flew into Denver, rented a car, had that car die at the rental checkout gate, got an upgraded replacement, and then put over 2000 miles on it, driving from pump station to pump station from Wyoming to Missouri and then Tulsa, OK and then back to Denver on a different line. Lots of dirt roads. Lots of red clay roads in Kansas (red gumbo when wet from rain, which in this case was all of them). When I turned the car in, you couldn't tell what color it was. They asked me where I'd taken it, and I said "everywhere". They were not amused.
I do NOT miss analog tone-operated telecom systems.