To notify the customer that their table is ready, you would have to click the "Signal" button next to the customer you want to notify.
After that, there will be a pop up alert if you want to notify the customer, just select "Yes".
Then another alert will pop up saying that they have successfully alerted the customer. This will be via Text Message.
Then the "Signal" button will have keep track of how many times you've tried to notify the customer and how many minutes ago you tried.