Here is two different ways as it could be for your phone, or you can just call your carrier and ask them to turn it off.
First...
Go to your home screen and tap on the phone icon at the bottom.
Once the keypad is launched, type ##002# and then click on the Call button. There is no cause for worry since the iPhone will not actually make the call, but will display the settings of your voicemail.
You will be presented with three different messages
* Voice Call Forwarding
* Data Call Forwarding
* Fax Call forwarding.
Do not do anything, but simply tap on the “Dismiss” tab at the bottom of the screen. You will have successfully disabled your voicemail for all types of calls.
Second...
Start by tapping on the phone icon found at your iPhone’s home screen. Next, enter the numbers *#62# then call the number so that you are able to determine your voice mailbox number. Note the number down on a piece of paper or memorize it if you are good with numbers.
Go to the setting section on your iPhone. As soon as the menu opens, tap on the phone icon and then go to the call forwarding section. At this point, type the voice mailbox number on the space that will be provided and then you can now exit after saving the settings.
Now, you can go to the keypad on your phone and then type the number #404 and then call so that you are able to turn off voicemail on iPhone. Afterward, you can try to call your voicemail box just to confirm if you have new messages.
You also have the option of enabling divert call on your iPhone in case you wish. When you have disabled the voicemail, you can dial #61# hence your phone will have a ‘no answer’ response if anyone is trying to reach you. You also have the option of dialing 67 for user busy response.
If you wish to reactivate the voice mailbox feature, you can dial 004# and call.
