I don't have the app on my fairly dilapidated android phone, but I have been told that some people have received messages via the app that they have been in contact with someone who has tested positive. It would be helpful if the app provided a date, time and location as my acquaintance says they have not been out of their own house, and they live on their own. On mulling this over I suppose it could be the postman droping of letters and parcels at the door. It's difficult to place confidence in the accuracy of the app enough in terms of coordinating contact data, if the assumed facts can't be verified by the receipient of the message. Self isolation because of an app message for some people is at personal financial cost, as not everyone can work from home, not all employers give paid leave, and statutory sick pay is a pitiful sum, assuming self isolation when not actual ill qualifies. In principle the app is a good idea to reduce the risk of contagious people spreading the virus inadvertently, but it also needs to be compatiblevwith everyone's phone and accurate.