How to find out my voicemail box number?

I have AT&T and I have an iPhone 4 and my voice mail has quit working. I went through the steps to reset my voicemail password so I definitely know that but when I go to reset my voicemail, the prompt first asks for which voicemail box you would like you use and I've tried my phone number, phone number + area code, and just the last four digits and all are "not recognized voicemail box numbers." How do I find out my voicemail box number?

Contact AT&T customer support.

You can find the number on your bill or the AT&T website.