That may depend on the exact set-up of the application or the technology because I think there are different solutions. I would have to get someone who is an expert in this to let you know.
I do know that, depending on the level of infection in the community, the number of contacts that you have to trace in order to suppress the chains of transmission can vary. It can be between 50% to 60% of contacts that you have to trace in order for that transmission chain to be stopped, so you can imagine that it takes a significant number of people to have signed up for this for contacts to even be notified.