Hi there and welcome to the forum. It's usually the last four digits of your home phone number.
You may be able to call your own phone number and then press the "*" key when the prompts start. The default password may be the last 4 of your phone number. Also in my area (Could be different in yours) I can call the following number:
(Area Code)1st 3 of Phone Number-9477
You can try calling your own telephone number and following the prompts to access your mailbox. You'll need to call your local business office to get your access number (check your bill for the number to call). The temporary starter passcode is usually the last four digits of your telephone number, except in some areas in NY and NE it is your 7-digit telephone number. You can't just plug in your area code with the digits suggested by the other responder. Each area has a unique access number.