how to check a phone number if register at whatsapp ??

if (!String.IsNullOrEmpty(this.txtPhoneNumber.Text))
            {
                try
                {
                    this.number = this.txtPhoneNumber.Text;
                    this.TrimNumber();
                    WhatsAppApi.Parser.PhoneNumber phonenumber = new WhatsAppApi.Parser.PhoneNumber(this.number);
                    this.identity = WhatsAppApi.Register.WhatsRegisterV2.GenerateIdentity(phonenumber.Number, this.txtPassword.Text);
                    this.cc = phonenumber.CC;
                    this.phone = phonenumber.Number;

                    CountryHelper chelp = new CountryHelper();
                    string country = string.Empty;
                    if (!chelp.CheckFormat(this.cc, this.phone, out country))
                    {
                        string msg = string.Format("Provided number does not match any known patterns for {0}", country);
                        this.Notify(msg);
                        return;
                    }
                }
                catch (Exception ex)
                {
                    string msg = String.Format("Error: {0}", ex.Message);
                    this.Notify(msg);
                    return;
                }
                string response = null;
                this.password = WhatsAppApi.Register.WhatsRegisterV2.RequestExist(this.number, out response, this.identity);

                if (!string.IsNullOrEmpty(this.password))
                {
                    //password received
                    this.OnReceivePassword();
                }
                else
                {
                    string msg = string.Format("Could not verify existing registration\r\n{0}", response);
                    this.Notify(msg);
                }
            }
            else
            {
                this.Notify("Please enter a phone number");
            }