Native Android and iOS apps using firebase to demo push notification.
A user is automatically assigned a unique numeric user id.
User registers the app by submitting phone# and verifying it via SMS.
When one user sends a text or picture message, the receiving app of the other user pops to the foreground displaying the text or picture message. User acknowledges the receipt of the message by tapping a button. If the user doesn't respond in 5 minutes, or if the user's phone is locked, a robot phone call is made, playing a recorded message when the user answers the phone.