For example Customer has a field of type PhoneNumber (value object).
In the persistence.xml a PhoneNumberConverter is registered which implements javax.persistence.AttributeConverter. This converter converts a PhoneNumber to string and visa versa, so the JPA provider is able to store PhoneNumbers into the database.
How to query a Customer with a LIKE operator on PhoneNumber with the Criteria API? PhoneNumber can only be a valid phone number. A PhoneNumber with a value like '+31%' is not possible.