For headless smallish servers (consumer hardware or SoC like Raspberry Pi) the sound interface is awesome because it needs no additional hardware (as with bluet

For headless smallish servers (consumer hardware or SoC like Raspberry Pi) the sound interface is awesome because it needs no additional hardware (as with bluetooth + software stack) and almost all mentioned devices have a sound card on board. I think it's not too hard to setup a getty instance in an old modem fashion with this. Additionally, as a client already a smartphone is suitable. This can be helpful in weird situations (think of non-working network).