I'm having in-law drama too. I've found that NC is the only way to go. If I'm not legally obligated to talk to them, I won't. I already tried the amicable route and it blew up in my face. Even after my MIL just insulted me last week she had the nerve to text and email when she mistakenly thought I was bringing DS to town this weekend. I wrote her back BUT DID NOT SEND IT. Had to get some things off of my chest but wouldn't give her or my FIL any more time or attention.
Is it possible to get your iPhone on your own plan? I know it's more $ but it might be worth it just to have what you want and revel at the fact that his dirtbag move has no effect on you.
I personally wouldn't fight it or even comment on it because that is what he wants. To start drama. Let him pay for a phone you don't use.