[4º Improvement]

Lead.cs

public class Lead : Entity
{
    private readonly List<Segment> _segments;

    public Lead(Name name, Email email)
    {
        Guard.Against
            .Null(name, nameof(name))
            .Null(email, nameof(email));

        _segments = new List<Segment>();
        
        Name = name;
        Email = email;
    }

    public Name Name { get; }

    public Email Email { get; }

    public PhoneNumber? PhoneNumber { get; private set; }

    public Address? Address { get; private set; }

    public bool? Gender { get; private set; }

    public DateTime? BirthDate { get; private set; }

    public IReadOnlyList<Segment> Segments => _segments.ToList();

    public void AddSegment(Segment segment)
    {
        Guard.Against.Null(segment, nameof(segment));
        _segments.Add(segment);
    }

    public void CompleteInfo(PhoneNumber phoneNumber, Address address, bool gender, DateTime birthDate)
    {
        Guard.Against
            .Null(phoneNumber, nameof(phoneNumber))
            .Null(address, nameof(address));
        
        PhoneNumber = phoneNumber;
        Address = address;
        Gender = gender;
        BirthDate = birthDate;
    }
}