There are two different ways. Via the web portal or by using the Avaya Cloud Desktop App (See Below).
Web Portal:
You will need to login to Avaya Central Office web portal at: https://service.cloudoffice.avaya.com
Then click Messages:
It will bring up your inbox for voicemail and faxes, Faxes will have the number of pages listed under the Message Column:
Hovering over the icon left of the number of pages will bring up a thumbnail preview, clicking the icon to the left of the number of pages will bring up a full-sized view:
Full Size:
Avaya Cloud Desktop App:
To Install:
You will need to login to Avaya Central Office web portal at: https://service.cloudoffice.avaya.com
Once you are logged in, click More:
Then Click Apps and Resources:
Then Click Desktop Apps:
Then Click on Download for PC:
Once it has finished downloading, click the carrot and then click open to install:
Once the application is installed, and you login with your credentials, you will be able to retrieve you faxes by clicking on Fax:
Selecting the fax will bring up a menu that will let you View, Download, etc.