Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
What phone service provider do you have, and have you had before you forgot the password? I think that this matters to the question, perhaps, or might.
OK. Here's some hope, though I'm not quite certain: Contact AT&T. They often provide voicemail with their service. This would mean the voicemail is not the phone's, but belongs to and is on AT&T's (distant, 'out there on the network') system.
Let me know whether I have guessed right - or not - please. Thanks.