When iPhones still had the headphone port, a friend of mine soldered a IR led on top of a minijack, something like this:

https://www.rtfms.com/wp-content/rtfms-com/LED-pinout.png

Then, with some special app, or even just playing some audiofiles — I don't remember — he'd do the same thing as the device above.

Wow, this is clever. Yeah, the headphone out can push out a signal like 1 volt at low current, but this is likely enough for the IR LED to "light up". I really like this idea.