FCM (Firebase Cloud Messaging) how to send to all Phones?
Solution 1:
Sending a message to all the phones like what you do from the Firebase Web Console is only possible from the Web Console. If you need this feature from the API you can submit a feature request: https://firebase.google.com/support/contact/bugs-features/
Another possibility is to have all the client registering to a specific topic via FirebaseMessaging.getInstance().subscribeToTopic(topicName)
In this way you can send a message to the whole topic without collecting the registration-ids manually.