Hi Maureen,
You arent missing anything here - We currently dont sync with the Mobile field on the Customer table for QuickBooks Desktop versions, as it has not yet been made available through the QuickBooks SDK (the platform we use to query and modify data from QuickBooks). When you try to add this field yourself - you're get a message telling you it already exists because it technically does exist on the table (in Method), but is only visible for QuickBooks Online users, where this information does sync.
If you already are using all the other syncing fields, you might want to look into using QuickBooks custom fields to sync this information across.
Thanks Maureen!
-Ben