How to change voicemail greeting on iPhone 5

So I had a sleep over with a bunch of my friends and they were goofing around with my phone while I was sleeping. They put a really inappropriate message as my voicemail greeting and my parents have heard it and I got in trouble. I tried changing it back to the default greeting, but it won't go. I really need help because I'm afraid someone like my grandma will call and hear it and tell my parents it's still there. I have an iPhone 5, not jail broken or anything.

I HAVE THE SAME PROBLEM I WANNA CHANGE IT BACK! SOMEONE ANSWER Please!