Provide a financial incentive for dropping your cell phone in a box. They would get an opportunity to use the phone during their break only. You keep the box.
I understand that cell phones in the work place are annoying and I agree that your staff should not use them during their shift, especially FOH employees. However, we all know that cell phones are a main source of communication, for some, the only form of communication. For instance, I do not have a house phone and haven't for several years and if something happens to one of my children or my wife, God forbid, my cell phone is the only way to contact me. You may say that someone could just as easily call the restaurant to inform you of an important situation, but if you are in the middle of service, the kitchen manager is not going to stop what they are doing and come get you to take a personal phone call.
This is a hairy subject. I would not take away the cell phones at the beginning of the shift because you, as the Exec. Chef / Manager, don't want to have to lead your employee to where the phones are kept every time someone goes on break. This wastes your time and theirs. Accordingly, I would not allow the employee to have the phone out and "in hand", so to speak, during service. Along with that, all phones should be set to silent/vibrate once they enter the building so as not to distract other employees.
This is one of those trust issues one has to conquer within their own comfort zone. I would allow the employees to carry them (again, except FOH) and ask that they respect your time and not dabble during business hours, but to also allow them to check their phone occassionally to make sure that they need to tend to a pressing matter. While building this comfort level, keep an eye out for those who are taking advantage and discipline the individual, not the entire staff.