@Entity
public class Person {

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @OneToMany(mappedBy = "person", cascade = CascadeType.ALL)
    private List<Phone> phones = new ArrayList<>(  );

    public Person() {}

    public Person(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public List<Phone> getPhones() {
        return phones;
    }

    public void addPhone(Phone phone) {
        phones.add( phone );
        phone.setPerson( this );
    }
}

@Entity
public class Phone {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne
    private Person person;

    private String number;

    public Phone() {
    }

    public Phone(String number) {
        this.number = number;
    }

    public Long getId() {
        return id;
    }

    public String getNumber() {
        return number;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }
}

final Person vlad = doInJPA( this::entityManagerFactory, entityManager -> {
    Person person = new Person( "Vlad" );
    entityManager.persist( person );
    return person;
} );

doInJPA( this::entityManagerFactory, entityManager -> {
    Phone phone = new Phone( "1234567890" );
    vlad.addPhone( phone );
    entityManager.persist( phone );
    Person person = entityManager.createQuery( "select p from Person p", Person.class).getSingleResult();
    assertEquals(1, person.getPhones().size());
} );