Most definitely add it in audio post - as always, filmmakers are control freaks. What's also fun is creating your own rings, ring-tones, etc. It can even be used as an inside joke, like using the music from an indie band that's in the film as a ring-tone. I created a text notification sound last week, as a matter of fact; I also created pseudo "Angry Birds" sounds since we can't use the real thing due to copyright issues.