Open the Phone app
Tap Voicemail
If your carrier supports it, contact your carrier to disable voicemail service
If you want to stop voicemail notifications, go to Settings
Tap Notifications
Tap Phone
Turn off Allow Notifications
If you want to forward calls instead, go to Settings
Tap Phone
Tap Call Forwarding
Turn on Call Forwarding and enter the number you want calls sent to
If you want to change voicemail settings, contact your carrier for account-specific options