One day we decided just to ban the phone outright, cold turkey. The tantrums lasted for a day or two then he forgot about it completely. I cant proove it was related, but we noticed an immediate increase in his level of vocabulary. Fast forward 4 years and we now use the phone only for calling granny and using the stargazing app at night.
PS We took this lesson and hit upon a great related strategy. We started to artificially "limit" book time with the young kids. And of course this just improves their excitement around reading! Scarcity in action :P