When you register with WhatsApp Cloud API, Facebook provides a Test WhatsApp phone number that will be the default sending address of your Application. For recipients, you will have the option to add a maximum of 5 phone numbers during the development phase without having to make any payment.
Later you can register your own Phone Number to avoid the limitation of 5 phone numbers.
TsgcWhatsapp_Client oClient = new TsgcWhatsapp_Client();
oClient->WhatsappOptions->PhoneNumberId = "107809351952205";