That's a nifty need gap. Missed call alerts which tell the number of rings or duration of rings would be definitely be useful, but as you say it's a feature at best.

This should be implemented by the phone manufacturers or operating system developers themselves as 3rd party apps has no access to the call features of both android, iOS nowadays for security reasons except for spam identification and calling app.

Perhaps, after market android ROMs and jailbroken iOS apps offer such feature; but that would mean a compromise in the security.