It was ubiquitous enough in urban and suburban areas in the US and Canada. The coverage of AMPS was solid, what was limited was the bandwidth and capacity - but there were far fewer users. Coverage in remote rural areas was poorer but still - in the late 80s it was being used for EMT transmittal of telemetry. And as stated using dedicated spectrum this is a 1970s tech. On rereading the OP I am more puzzled what they are talking about because it seems like they are talking about EMT monitoring as opposed to continuous home monitoring - the former we have been doing for 50 years.