How do I fix my phone which keeps buzzing and going off every 20 secs

Check Your Notification Settings

Even if Vibrate mode is off, one of your apps could be trying to tell you something. You can add specific notification settings for each of your apps and set your phone to vibrate when you receive a notification or message.

Navigate to Settings, and go to Notifications.

Go to Manage all and check your notification settings for each app installed on your device.

Tap Notification sound and check if you set your terminal to vibrate when you get a notification from that app.

Regards,

Will