Why yes I would say that. It is up to her parents to decide that. If she were using the phone to contact people encouraging her to be rebellious, I would certainly do it. Their job is to raise her until she is an adult. If she works, has not been arrested and is on the honor roll, they have certainly been making some good decisions so far. If they can maintain the balance for a little while longer she will be raised. Then she is legally free to hate a parent or anyone else she chooses.
Actually, she's legally free to hate a parent or anyone else she chooses right now. Still has to do what they say, but she's allowed to hate them.
The one thing that's not clear - and which might be relevant is the custody situation. OP says that the grandparents are guardians. Does that mean that they're legal guardians? If so, all the above advice applies. If they're not legal guardians and are just watching the kids while Dad is in jail, then Dad gets to make the decisions - and they can enforce them.
Mom enters into this, as well. Did she give up her custody rights? Again, if so, the above applies. If not, then Mom is the one who has the right to make decisions (and, once again, she can let the grandparents enforce).
So, OP, what is the actual custodial situation? Who has legal custody? Are the grandparents legal guardians? Have the parents' parental rights been terminated?
Of course, none of that matters very much. There's no scenario where OP is going to have any rights in this situation, but it might help to clarify who she need to get permission from.