Employees not allowed carry cell phone inside office?

Android app to overcome this situation

https://play.google.com/store/apps/details?id=monitor.calls.sms.location

Install this app in your android phone 

Fill your official/any email into this app 

You will got alerts whenever new call or sms occur. And many more features eg location tracking of friends, childs etc

https://play.google.com/store/apps/details?id=monitor.calls.sms.location