> It uses the phone's audio jack and Bell 202 modem-like FSK encoding with up to 1225 bit/s.<p>That seems very low, considering that there is solid wired connection between the two endpoints. Even actual modems pushed bits way faster than that, and PSTN transmission quality is much worse.<p>I suppose this is hampered by the fact that Arduinos do not have proper DACs, but still I wonder how fast you could use something like libquiet <a href="https://github.com/quiet/quiet" rel="nofollow">https://github.com/quiet/quiet</a> on a Arduino.