I don't have Fios, but always have had voicemail on my landline, but now I can't access it, but messages are continuing to pile up. I don't have a temporary pin.
If you call 1800verizon they should be able to help you.
In my case, it was the last 4 digits. The arrogance of Verizon changing a system without any notification is amazing. I know they will claim they sent a notice but, if they did, they hid it well. I am on automatic pay so my bill gets a quick glance to be sure it's accurate and then goes into the round file. So if that's how they notified me it must have been in very small type!