It isn't really an issue they can fix. Voicemail and missed call notifications come in from carriers over specially formatted messages. You'd need to develop a

It isn't really an issue they can fix. Voicemail and missed call notifications come in from carriers over specially formatted messages. You'd need to develop a new out of band messaging protocol that is as reliable as SMS delivery and get all the 30+ carriers Apple works with globally to roll it out before they could turn off the old stuff.