Just like it sounds: a silent ringtone for iPhone.
I created this repo because I like setting up my iPhone to "vibrate only" when sound is turned on, and "truly silent" when in silent mode (so my phone never rings or beeps in any way). To do this, you need to turn on "Vibrate on ring" and have a silent ringtone. However, a (free) silent ringtone is just about impossible to find.
To set this up for yourself, clone this repo or download Silence.m4r. Transfer the ringtone to your iPhone (you can drag and drop it into iTunes, for instance), then set your ringtone to "Silence." Turn on "Vibrate on ring" and turn off "Vibrate on silent." Then just make sure all other notifications are set up to vibrate only.