I agree about warning your ex H in very general terms. You need to practice preemption in this case.

Go to a retail outlet of the cell phone provider company and remove yourself from the joint account and get your own phone number. You need to sign papers, which is why I suggest you go to a retail shop.

And in your future relationships, keep your own phone line until you get very deeply involved together - it does not cost THAT much money to have a separate account and the premium it costs is well justified by having your privacy and control.

Likewise, do not share your passwords.