My dear Ajay,
When this is a Policy decision, then we must be very strict on this. In my organization also people were initially allowed to take cell phones inside the Plant and suddenly a decision was taken to ban it. In such cases, those who have been used to it, would either protest (through Unions or individually) or would take inside concealing in many ways. Talk to the Unions first on this and take their co-operation which we also did. The sign boards will also be necessary all over the plants.
The security checks should be vigorous and staff / supervisors at all shop floors should be entrusted the responsibility to watch and inform the offenders. This is what we do - after a few warnings, we now impound cell phones. You know what they are doing now, once they take the cell phones inside and keeping them in their pigeon holes. They take the SIM alone inside and use!!! We have then ordered the Security to make surprise checks in their pigeon holes.
Initially it will be difficult and once they realize the co-operation of the workers will be there.
All the best.
Ganesh