Select Into will only work when the table doesn't exist. You can use the following SQL to insert data into existing table.
Try this -
Insert into Total 
 select customernumber 
    from ccsowner.customer 
    where customernumber not in (select customernumber from MOBILEACCOUNTDETAILS)
If you want to use the SELECT INTO, you could use the following SQL (It will create Total table on the fly and will give error if Total table is already existing):
;with cte_Cust As (
    Select customernumber 
      from ccsowner.customer 
      where customernumber not in (select customernumber from MOBILEACCOUNTDETAILS))
Select customernumber Into Total
   From cte_Cust