NULL vs empty string in SQL Server

NULL is unknown. A blank is a value. An empty-string check is false for NULL. Reports that should say No phone need to catch both.

Treat both as missing

SELECT customer_id, first_name,
       CASE WHEN phone IS NULL OR LTRIM(RTRIM(phone)) = '' THEN 'No phone'
            ELSE phone END AS phone
FROM Customers;

Or NULLIF(LTRIM(RTRIM(phone)), '') which turns a blank into NULL, then ISNULL.