Can we tap company phones to investigate possible theft?

Q. Our company distributes steel from one of our warehouses. A substantial amount of product has gone missing. We suspect that our warehouse manager is conspiring with one of our former employees to take it, possibly creating a false paper trail to cover the theft. Our camera surveillance is not picking up any irregular activity on the loading dock. We believe the manager may be talking to the ex-employee from his office phone. Can we legally monitor the manager’s phone without violating any privacy rights that he might have? ...