Apple/Google APIs
The main challenge with contact tracing is keeping the relevant sensors always on to detect the presence of others. Most iOS and Android end users won't care about the impact to battery life and filesystem storage but I do. My vote is to have every single device forcibly install an optional contact tracing database function as a critical system update which is enabled by default BUT with a simple off switch for those wishing to save their device resources rather than the 'greater good' of society. If 99.9% of users choose to accept the tracking then fine but at least provide an off switch, similar to those weird folks who don't even use a smartphone :)