A push notification is sent either through Apple or Google servers. If a device has not registered with Apple/Google and then with Backendless, it would be impossible to deliver a push notification to the device.
In ANY app a push notification can be received ONLY because the device has registered with the provider. If you register device with Backendless, then you can send push notifications. Phone number authentication is absolutely irrelevant here.
Let’s forget about push notifications for a moment. Could you describe what you’d like to accomplish in your app? It would be the best if you could describe it not from a technical point of view, but rather from the user experience.
If you select a user from the address book, there are two possible scenarios:
The other user already has your app installed
The other user does not have the app installed.
If they do not have the app installed, what should happen then? The system sends an SMS saying that someone invited them? What will happen next?
I think I understand the source of confusion. Push notification and SMS message are two different things.
When you send an SMS message, it is not a push notification. In fact, it is a completely different type of delivery. Here are a few articles describing the differences:
SMS messages can be sent with a provider like Twilio (which Backendless integrates with). To send an SMS message, a device does not need to be registered with the backend, all you need to know is the phone number:
Push notifications require device’s registration with the backend.