Missed call notification
If we can detect somehow when a call went unanswered, but there is no recording (or only a blank/useless recording) send a "Missed call from ..." message so the user at least knows someone tried to call.
Most users will get this from their phone or app where the call went before going to voicemail, but still nice to have notifications all in one place, or nice for users like me who don't normally have their phone ring anywhere.