Legal Question in Family Law in California

Only if you want to have that thown in your face in the divorce. While technically not illegal, being vindictive like that is a really good way to get the court on the other person's side. The proper way to handle it is to give her reasonable notice that she needs to put her phone in her name. Cell companies process that kind of thing all the time, and most of them don't even count it as a contract early cancellation if you both continue to use the same carrier. Then if she blows you off about changing the service, you can cut her off and she's the one playing games.