Sometimes, an iPhone asks for a voice mail password repeatedly, even though it hasn't requested the code in the past. This may happen when you're trying to access your messages or at times when you aren't even using the voice mail system. Check your password and reset it to see if that fixes the problem. If it doesn't, you may have a glitch on the iPhone that you may be able to fix with a restart, reset, update or network connection refresh.
Check and Reset the Password
Check you're using the correct password to avoid repeated prompts for a password. If you don't remember the password or don't remember ever setting one, try resetting it. You can do this from the iPhone even if you don't know the current password. This process may also help if you're sure you're using the correct password, but the iPhone won't accept it or keeps asking for it after you've entered it correctly.
Tap Settings and then Phone.
Tap Change Voicemail Password.
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Enter a new password in the screen and tap Done. Reenter the password on the next screen to confirm it and tap Done. The iPhone saves the new password.
Restart and Reset the iPhone
If a glitch is making your iPhone continually ask for a voice mail password, it may fix itself if you switch off the handset and restart it. If this doesn't fix the problem, try a reset. Neither fix should affect your data, and either may clear the glitch off the system.
Restart the Handset
Press the Sleep/Wake button and hold it down until the Slide to Power Off message appears. Swipe the slider to the right. When the iPhone powers down, hold the Sleep/Wake button to restart.
Reset the Handset
Press the Sleep/Wake and Home buttons simultaneously and hold them both down until the Apple logo shows on the screen. Release both buttons and wait. The iPhone will restart once it has reset.
Check for Carrier or Operating System Updates
You may have a problem with connections or settings if the iPhone's carrier or iOS software is not up to date. Check both settings in iTunes.
Connect the iPhone to iTunes. If a carrier update is available, you typically see a message as soon as iTunes recognizes the iPhone. Select Download and Update in the message window to update the iPhone.
Select the iPhone's icon from the menu and select Summary.
Check the summary area on the main screen to see if an iOS update is available. If one is listed, select Update. If no update is there, double-check by selecting the Check for Update button.
Reset the iPhone's Network Connections
Resetting the iPhone's network connections may clear a glitch or fix a settings problem. This process takes the iPhone's network connections back to factory defaults, deleting its current connections. After the reset, rejoin your networks. You'll need network passwords -- such as Wi-Fi codes -- to rejoin, so make sure you know these passwords before you start the reset.
Open the Settings menu and tap General. Tap Reset.
Tap the Reset Network Settings button.
If prompted, enter the iPhone's general password. Tap Reset Network Settings to start the process. The iPhone resets its connections and restarts.