I've been on the internet at home since the late 1990s but always with a desktop. Neither my wife nor I have any desire to have a mobile phone other than the basic pay-as-you-go model that we take out with us in case of emergencies but otherwise never switch it on. It's not connected to the internet. As has been said there seems to be a basic belief in society that everyone has an internet-connected mobile phone.
As to the system, one of our grandsons accompanied by his Australian girlfriend flew into the UK from Melbourne over a month ago. Because they transited via Qatar they had to quarantine for 14 days. A week or so later our son and daughter-in-law returned to the UK from a holiday in Turkey and also had to quarantine for 14 days.
Neither our grandson, his girlfriend, our son or our daughter-in-law was contacted at any time by anyone checking that they were abiding by the quarantine.
Perhaps they were in a minority and it's just a coincidence that none of them were contacted. Perhaps!