How to Turn Off Voice Mail on an iPhone
Disabling iPhone voice mail can be a difficult process, though a worthwhile venture for those traveling abroad wishing to avoid exorbitant roaming charges. AT&T suggests disabling "data roaming," though this alone may not be enough to sidestep fees for incoming voice-mail messages. Users may choose to forward calls to the iPhone's voice mailbox, where they can be retrieved later, or to deactivate the voice-mail feature altogether.
Discover the number of your voice mailbox. Type "*#61#" without quotes and press "Call," then write down the number the service gives you.
Forward calls to the voice mailbox number. Go to "Settings | Phone | Call Forwarding." Set the voice mailbox number as the number to which calls are forwarded.
Disable the voice mail. Type "#004#" without quotes and press "Call." This should deactivate your voice-mail service. You can also choose to forward calls to a different number than the voice mailbox, or selectively enable and disable diverts for "no answer" (type "#61#" without quotes and "Call"), not reachable ("62" without quotes), or busy ("67" without quotes).
Reactivate voice mail:Reactivate voice mail by dialing "*004#"" without quotes and pressing "Call."
Call AT&T customer support and request that they deactivate your voice-mail service if you need help doing this.