No Further Entries Of A Value In Table

I have a table which stores phone numbers of a customer in a table.

Say this table is as below

CustomerName - PhoneNumber
Customer 1 - Phone number 1
Customer 2 - Phone number 2
Customer 2 - Phone number 3
Customer 3 - Phone number 4

What would be the best approach to prevent adding another entry against Customer 2. I should be able to add new customers and add multiple phone number against all other Customers. The restriction should be only against Customer 2.