If someone is calling your child from a number that you haven't previously approved on your app, those calls will get blocked and will not manage to reach the watch. You, on the other hand, will receive a notification on the app notifying you about a 1 sec call - the feature is there to let you know that a stranger is trying to contact your child.
If the calls are from someone your child knows, you can save their numbers on your app, so that they can start calling each other. We cannot unfortunately block the notifications from our end, as it is meant to be a security feature. If you don't know the person who is calling and you'd like to block the calls and notifications on the Xplora App, you can check with your provider if there is anything they can do on their end.