I am not sure why you are defining these data items in the linkage section.
Data Items defined in the linkage serction have no actual storage assigned to them.
The storage is assigned either by passing parameters to the method from a calling program using the procedure division using header, or by using set address of <linkage-data-item> to ...
In order to reference these data items as sending or receiving data you must first set them to a valid storage area in working-storage or local-storage.
You could also modify the method to use:
procedure division using customer-name customer-address customer-phone customer-mobile.
and then pass these parameters in the invoke statement that calls this method.
invoke myobject::addCustomer(customer-name, customer-address, customer-phone, customer-mobile)