I find that vibe makes it a little too easy to miss calls amd messages: the device might be in a loose pocket, a bag or on a soft surface, and the buzz is missed. The compromise is a ringtone—of sorts. I nicked the first couple of chirps from the Star Trek (TOS) annunciator sound effect. It's easily distinguished from other sounds, penetrating and recognisable but too brief to be annoying to anyone. By the time most people are wondering if they actually heard anything, it's done: but I know my phone farted.
I suspect there may be a small piece of modest but worthwhile undergrad work figuring out what kinds of annunciator effects meet the criteria for 1. brevity, 2. low volume, 3. penetrativity without aggravation.
Once the essential characteristics are understood, there's conceivably a patent (certainly in the US, where you could get a new patent for the wheel) but certainly a niche business opportunity: an app which crafts 'chirpies' to individual taste so that in the office, everyone will know and recognise their own almost-subliminal chirp while ignoring others'. I'd guess that combinations of pitch, rise and fall, mutli-tone, spacing and intensity should allow at least a hundred recognisable combos. The human ear and brain are pretty trainable.
Send my 10% of sales to GOSH children's hospital.