I don't have a smartphone either, but I was trying to imagine what it would be like to have to go into full isolation when there was an 80% chance of it being a false alarm.
And what about workplaces - each time a member of staff gets a cough, the whole workforce might potentially have to isolate, so they'd probably have to close down until the test results came back. 80% chance of going back to work then - and next day someone else gets a cough. Repeat ad nauseam...?
OK, we've got to get out of lockdown somehow, but is this really the best way to do it?