Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Since, regardless of what device you access it from, voicemail tends to be actually 'out there' on servers or company machines, you need to contact ATT (or whoever your voicemail service is with) and first
- cite your account number
- identify and verify your identity as required
- then ask them to reset your voicemail password, or otherwise solve your problem.