Frank, I hope you realize that her attitude has more to do with the disquiet in her soul than it does with you.
Leave her cell phone alone.
You don't need anything else to deal with and you also don't need to give her a reason to turn to the kids and suggest you are the bad guy because you are throwing her out.
Maybe shelve the issue for a couple weeks at least.
Or
If she doesn't pay for her own phone, shut it off. She can get a tracfone or something else where she can pay as she goes.
Under the circumstances, it is not unreasonable that you would decide to trim some of the fat.
