PhoneNumber.cs

public class PhoneNumber
{
    public PhoneNumber(string ddd, string number)
    {
        Guard.Against
            .NotMatch(ddd, @"^\d{2}$", nameof(ddd))
            .NotMatch(number, @"^\d{8}$|^\d{9}$", nameof(number));                                   

        Ddd = ddd;
        Number = number;
    }

    public string Ddd { get; }

    public string Number { get; }
}