Can I check if a user's phone number is enabled for WhatsApp?
Currently, we do not offer an API to check if a user's phone number is WhatsApp enabled. WhatsApp requires businesses to collect an active user opt-in, in order to message them via WhatsApp. Hence, we recommend that you ask users if they have a WhatsApp enabled number.
Sending a WhatsApp message to a phone number that is not WhatsApp enabled will fail and fallback to SMS.