PhoneNumber.cs

public sealed class PhoneNumber
{
  private readonly string _;

  public PhoneNumber(string s) 
  {
    if (!ValidPhoneNumber(s))
    {
      throw new ArgumentException("Not a valid phone number");
    }

    _ = s;
  }

  public override bool Equals(object that)
  {
    return 
      that != null &&
      this.GetType() == that.GetType() &&
      this.ToString().Equals(that.ToString());
  }

  public override int GetHashCode()
  {
    return _.GetHashCode();
  }

  public override string ToString()
  {
    return _;
  }
}