New orders on restaurant tablets for a delivery app

Your food delivery app sends each new order to the restaurant's tablet, where staff tap Accept. About 5,000 restaurants use it. The dinner peak brings 400 orders a minute across all of them. The tablets sit on busy counters on the restaurant's Wi-Fi, and some drop offline for minutes at a time.

A customer should know within 5 minutes whether the restaurant has taken the order. The tablet app tells your servers when an order is on its screen. If an order has not shown on the tablet within 1 minute, an automated phone call reads it to the restaurant. Staff can accept on the call by pressing 1. If nobody accepts within 5 minutes, the order is cancelled and the customer is refunded.

How does an order get onto the tablet, and what decides when to call and when to cancel?