Open the Phone app
Tap Voicemail
If you want to stop voicemail completely, contact your carrier and ask them to disable voicemail
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 of voicemail, go to Settings
Tap Phone
Tap Call Forwarding
Turn it on and enter the forwarding number
If you want to silence voicemail alerts only, open the Phone app
Tap Voicemail
Delete any saved voicemail messages
Contact your carrier if voicemail still remains active