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.
