I think you're right, USNA1985, on a company to company basis, of course. I think some plebes in some companies had iphones, but were not to use the music players.
Plebes were issued computers, obviously, that were capable of playing videos and music that they don't "rate" as well. They are not to use those features (and not getting speakers with the computers increased the likelihood that they wouldnt!).
Getting fried for listening to music on an iphone is not worth the 3 minutes of fun it would bring. By second semester, some companies, though certainly not all, backed off of the "no media for plebes" rule. My son's company was NOT one of those.
That's probably as specific as we can get. A current mid may clarify further, but I think it's going to vary from company to company.
I think a parent could GET their plebe-to-be an iphone, with full knowledge (on the part of parent and plebe) that they may be forbidden to use the music features while not on liberty or leave. JMHO.