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