I suggest that anyone who has the ability to record voice memos and use them as ringtones to do this, especially if you're like me and have professors who will rain down an ungodly terror on you should your mobile device erupt during they're boring lectures.
Go to a quiet place, background noise will be dangerous if you have a tendency to leave your phone on a high volume. Sure a cough is pretty normal in a classroom or church, but the pomeranian in the background may be a dead give away that you're not just scratching your leg when you frantically attempt to quell the disturbance.
Don't cough into the phone, but near it. You push a lot of air out when you cough into your mic piece, which will distort the sound and make it sound more mechanical. A faint cough sound is less noticeable to a teacher than the loud unnatural noise of air into a microphone.
Lastly, test it, make sure that you're cough sound is something that will sneakily alert you that you have a text, you left your sound on, and you're an idiot, without letting EVERYONE know that.
Just a little advice for the forgetful.