Cell phones in plant: Must we bargain with union?

Q. Our company has a union contract with work rules. We also have the right in the contract to change the work rules, which the union can grieve. We recently exercised our right to add a new rule prohibiting cell phones in the plant. The union hasn’t filed a grievance, but it has filed an unfair labor practice charge with the National Labor Relations Board. It claims that we are obligated to bargain over the new rule. Are we obligated to bargain over a new rule like this?—R.S.