Would it be illegal for a person to destroy a phone you paid for and gifted to someone else?
Example: Giving a phone to a friend then their mother destroys it or takes it away.
Maybe, though even if it is it might be a civil wrong and not a crime. But even if is a crime or a tort, the victim would be the person you gave the phone to. You no longer own the phone, so you have no legal standing to complain about what happens to it.
Note that the mother's rights are broader if the child is still a minor. If the child is an adult, the mother's rights are much narrower.