There are now different systems to track potentially infected users of phones, some even in a privacy-preserving way: https://www.pepp-pt.org/ and https://github.com/Co-Epi. Both systems rely on the bluetooth signal strength to estimate if two phones are near to each other. But this strength varies a lot depending on the position of the phones, if they are in a bag or pocket, and more.
Based on https://asa.scitation.org/doi/pdf/10.1121/1.4860156 one can improve the system by not only detecting the frequency, but by encoding a preamble and then searching for that preamble.
What it does
Measure the distance between two phones using audio.
How I build it
I propose to do a proof-of-concept using nativescript, because that allows to directly test it on Android and iOS.