If you are making the argument that mom owns the account, then you are likely correct.
Was the account stolen?
Want to bet the police figured that out before they made the arrest?
Don't know that they cared who owned the account. The physical property was what was alleged to have been stolen. Whether the mom continued paying the bill on the account is not really relevant to the father taking physical possession of the phone.
They better care who owned the account and who's paying the bill.
If my name is on the contract and I'm paying my kid's cell phone bill, I own the account AND the phone, no matter what my kid wants to think.
I'm not necessarily saying ex-wife was blameless. But, if it's hers (Mom's), ex-husband must give it back to her (Mom). Put it in a box and ship it to her! Problem solved.
Because yes, while the minor child is in Dad's care, he has every right to forbid her the use and possession of the phone or anything else, no matter who owns it.
But, this is just the sort of sh*t that happens when adults act like infants.