While migrating a program from EWS to Graph that adds/modifies contacts (including office/mobile/fax numbers) for specific office365 users, there's no way to add/modify a fax number for an office365 user contact in Graph
In addition to:
Contact.BusinessPhones
Contact.MobilePhone
We add the following:
Contact.BusinessFax(es)
Or something similar, or some other way to access/modify Fax number in office365 contacts from Graph
Sourced from https://github.com/microsoftgraph/msgraph-sdk-dotnet/issues/1325