If you have a product or business that maintains user information like phone numbers, verifying the validity or ownership of the phone number could become important, as the phone number can be used as an authentication method or targeted marketing channel. The typical phone verification procedure is by generating a code or OTP in our application, sending that OTP to the user's phone, and then the user should insert the OTP in our application for verification. The OTP can be sent to the users through services like SMS or WhatsApp that require a valid phone number. For internet-based communication, WhatsApp has become the de facto standard for sending the OTP. WhatsApp requires its users to have a valid phone number during account creation, and it already has a huge number of users, approximately 3 billion in 2025. Using that common procedure, WhatsApp will charge us for each OTP sent. The cost depends on the country of the target phone number. For Indonesia...