By default, the fax number variable on fax cover pages is the number that is displayed for the user in the "Fax" -> "Fax number" section of the XPhone Server (in certain constellations, the number displayed in the "Fax. Service" field). What needs to be configured so that the fax number from "Fax. Service" and not the fax number from the "Fax Number" field?
Go to SYSTEM SETTINGS -> UM -> FAX -> GENERAL -> "Advanced settings". Add a new parameter named "Mail2Fax\SFAXFromMasterData" (selectable from list) and set the value to 1.
Save the setting twice and restart the marked UM services.