No, personal 'phones of any sort are not allowed on Op HERRICK. There are good and sensible reasons for this, mainly to do with casualty notification - but also, and nearly as important, in order to stop journo trolls from wheedling info out of Soldiers and their families.
He'll probably have reasonably good internet access, and welfare 'phones are (apart from in some of the FOBs) pretty easy to use as well. Whether he'll be able to call at a regualr time, or at one that's convenient for you, is another matter entirely
I'd suggest getting him a decent pen, so he can write to you using the free Bluey system. It's much nicer to get a letter - and you can keep them as well, my Missus keeps those I sent (probably for use in legal proceedings, knowing her).