In order for the calls to come through, push notifications must be turned on. The good news is because it was never actually called, it's still in your APP to reschedule. If you are going to schedule it for another day/time, please be sure to slide the notification to open the App, and you can get your child ready for Santa's call!
Here is an example of what you will see when you log into your Santa Call APP.
Below is what appears when a call is originally set up. If notifications are not allowed, then the notification can't notify you to swipe open the APP. If notifications are allowed, then it's a matter of sliding the notification open which will then open the APP and allow you to confirm the call and then receive it.