This idea has been merged into another idea. To comment or vote on this idea, please visit CUSTCOM-I-1228 Ability to assign Custom Fax Cover Page to all Users with Extensions (Company ).
Basically, the phone app lets you create custom fax cover pages and upload them.
However it's only usable in the phone app (Note: not the android // iphone app just the ringcentral phone app).
Please spread this functionality to other platforms and then let us set through the admin portal for people to use this as default.