This is still a research project, and might require talking directly to phone OS developers like Apple and Google to implement properly. It does work cross-platform, though, and MIT is also showing its system to US federal and state governments.
If PACT can be ready in a timely fashion, it could be useful not just for minimizing the spread of COVID-19 during the outbreak, but play a vital role during the recovery. When lockdowns start lifting, an automated system like this could help keep public spaces open by asking people to quarantine only when they’ve recently been close to an infected person. Of course, this also requires widespread adoption to work — it would have to be an integrated, mandatory part of the OS to ensure that the right people know to isolate themselves.