Hello,
I'm sorry you are having issues with your device.
I think the reason tech support couldn't help you is because the voicemail password is not configured on the phone and it cannot be recovered from it.
Your first step is to find out what is the actual system that is holding your voicemail, it could be a local PBX, or you can be using a device called an UC500 or an UC300, or the voicemail could be hosted online.
Before you can proceed that is the first think you need to find to, what is the actual device hosting the voicemail. Please remember, there is no way the phone itself can hold your voicemail and the configuration for it s not inside the phone either.
I hope this is helpful.