If you have fios digital voice the you go tp www.verizon.com/fiosvoice and log in with the same user/pass that you use to check your bill. If you have regular fios phone or regular landline service then you have to sign up for verizon call assistant. Go to www.verizon.com/callassistant.
You also have a number you can call and you can access your messages from the phone. Fios digital voice is 888 2 FIOSVM. I thin that's 888 234 6789
If its regular fios phone or red landline then there are local numbers that you should be dialing and their repair department can give you the number. 8885531555. Its different for every area