What happens if I write a message to a phone number that doesn't have WhatsApp? Your messages will only get delivered to contacts having the WhatsApp app. Since messages are not delivered to phone numbers that do not have WhatsApp, you won't be charged for them.

If you send a message from within the WhatsApp app to someone who does not have a WhatsApp account, the message cannot be delivered via the app. Instead, WhatsApp will prompt you to "Invite" the contact to the platform via a standard SMS text message. However, if a business uses a professional WhatsApp API, they can often configure an SMS Fallback system. In this scenario, if the system detects the recipient doesn't have WhatsApp, it automatically re-routes the message as a traditional SMS text so the user still receives the information. For a regular user, if your friend isn't on the app, you simply have to switch to your phone's native "Messages" or "iMessage" app to reach them. The two systems are not "interoperable"—you cannot send a WhatsApp message that appears as a blue or green bubble on a non-user's phone without the sender manually switching apps.