Open T113 in Designer and add new field (e.g. 50000 - Customer Name; Text-50). Open propertiy of this field; set FieldClass as FlowField. Set CalcFormula as 'Lookup("Sales Invoice Header"."Bill-to Name" WHERE (No.=FIELD(Document No.)))'. Compile table and this is all about this field. You need to make the same for Customer Phone etc.
After that, you need to add this new fields in form.