Here's how it works: Let's say an unknown person sends a text to your child's phone. If their contacts are stored locally but not in the Cloud, the Messages app offers to add that person to the contacts. If your kid does that, the number is added and they can call, FaceTime or text the individual.
... On top of that, CNBC found that a child can ask Siri to call or text any number on an iPhone or Watch, bypassing the Screen Time limitations.