Task: Text to Code
Input (English): Step 1: For each row in Customers table, find the corresponding rows in Complaints table.
Step 2: find the number of rows of each value of Complaints's customer_id in the results of step 1.
Step 3: find phone_number of step 1 results with largest value in the results of step 2
Output (English): SELECT T1.phone_number FROM Customers AS T1 JOIN Complaints AS T2 ON T1.customer_id = T2.customer_id GROUP BY T2.customer_id ORDER BY Count ( * ) Desc LIMIT 1