How to change my voicemail on my iPhone 5?

Leo
29.11.2015

I have an iPhone 5 and I'm using the same sim as my old phone and I want to change my voicemail but i don't know how, I have the newest update aswell

Lauren
30.11.2015

Call your phone number and enter your password then press the * key

If i change my number and someone calls my old one will it go to voicemail? I'm on 02 and have a contract Iphone. I'm asking them if i can change my number but if i change it and someone calls my old number will they hear my voicemail or will it say can't be connected etc.
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How to change voicemail on IPhone? When I don't answer my IPhone I want a custom message to be said, but whenever I go to Phone>VoiceMail it takes me to a dial where I put a password. I've tried everything I could think of for the password but still does not work. I use AT&T for my phone service if that makes any difference.
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.