Here's another idea: TCP over vibration. Put one phone on top of the other, the bottom one in vibration mode, the other using its accelerometer to receive.
Interestingly enough, me and that very same friend from my prior comment also joked around with this idea. We never implemented it however, after I vaguely remember reading about a POC in some article from an engineering journal...
Not only are accelerometers precise enough for data transfer, me and my colleagues demonstrated data transfer at ~200 bps (after error correction) through the user’s finger to a wristworn smartwatch. Take a look: https://www.robertxiao.ca/research/viband/
This paper investigates the possibility of communicating through vibrations. By modulating the vibration motors available in all mobile phones, and decoding them through accelerometers, we aim to communicate small packets of information. [...] We develop Ripple, a system that achieves up to 200 bits/s of secure transmission using off-the-shelf vibration motor chips, and 80 bits/s on Android smartphones
How? A naive solution would use only "upwards acceleration" and "lack of upwards acceleration"; phone accelerometers should be able to handle that quickly enough to get at least couple dozen bps.
