You PIN is a four digit number that is used anytime you check your voicemail from a desk phone or by calling your Dialpad phone number. You can view or change your PIN by logging in at https://dialpad.com and finding the section for User & Voicemail PIN.
Many people never use their PIN, because they read or listen to their voicemail in their email. Although, checking your voicemail in your email will not clear the Message Waiting Indicator light.