Presently, when you use the Fields property, you can only access simple type properties : "= Fields.CompanyName", "= Fields.CompanyPhone". The complexe types are not accessibles via the Fields property. If you want to print a complexe type property, you need to use the expression "= ReportItem.DataObject.Company.Name", "= ReportItem.DataObject.Company.Phone".

In my opinion, this is not very intuitive. 2 differents ways to read a property depending of his type! It would be great to be able to access anything from the Fields property.