To my knowledge, there is no regulation governing the use of cell phones in military environments unless it is a classified area or an area in which a cell phone would create a safety hazard. For example, you cannot bring a cellphone into a classified briefing or to an area where explosives are being prepared.
If neither of those conditions exist, you probably cannot restrict a Soldier from bringing a cell phone to work. However, if the Soldier’s cell phone overuse is preventing him from being able to do his job to standard, a performance counseling can be administered. You could recommend to the commander that the Soldier’s cell phone privileges be revoked for a period of time to get the Soldier to comply with reasonable use. Separation action can be initiated if performance does not improve.
Consider establishing a work area policy that dictates that personal calls will not be allowed except during an emergency or that restricts use to a limited number of minutes per a given number of hours (somewhat like a smoke break). A policy of this nature would need to be reviewed by the command and potentially JAG.