A radio on a soldier is already a dangerous communications device - with a radio you can call in artillery strikes, for example.
There's no particular need IMO to secure smartphones on the battlefield in anyway beyond standard counter-measures - i.e. encrypt the storage, use a passcode unlock.
That's referring to people literally posting selfies online (with the result of giving away their location by either metadata or geo-guessing).
Which is a process and procedure issue, more then a security issue on the phones themselves (except in so far as it's really obvious there's a solid need for an OS for a battlefield device which strips all that stuff out by default).
There's no particular need IMO to secure smartphones on the battlefield in anyway beyond standard counter-measures - i.e. encrypt the storage, use a passcode unlock.