Depending on the RDBMS you are using you may be able to qualify the table names with the database names.
So you could use the following SQL:
SELECT COUNT(Cust.phone_no), Sup.local_carrier
FROM customer.customers AS Cust, supplimental.table AS Sup
WHERE Cust.CarrierID = Sup.CarrierID
GROUP BY Sup.local_carrier
You may want to check my syntax, however the theory is there.