Hi. Just call for a meeting if the numbers are less, else send an email stating that whoever wants to attend to personal calls on their mobile may please do so at the designated place but put logout and login in the time-sheet. See that you have a time-sheet in place in the system and people enter them, [as most of the software companies do, in order to give accountability to the clients], this would deter the employees to an extent though not a solution in itself. Staff need to have discipline though one or two calls a day is quite negligible instead of putting a blanket ban. If the call exceeds 20 or more minutes in a day and this happens on a regular basis, the reporting manager has to talk to the concerned staff and if required send a letter to cut down on the calls/fiddling with the phones. Tread carefully though it needs to be taken head-on if the situation demands.