The Default Voicemail numbers are:
Internally from any extension on the PBX (User will be prompted for their extension number and password).
Internal from the extension of the User checking Voicemail (the user will only be prompted for a password).
Get to your voicemail prompt by dialing a DID, or through an Auto-Attendant (AA). Once you hear your voicemail greeting press star(*) to be prompted for your password.