Yes, they can. Unless this mandate violates the terms of a union agreement or employment contract, then as an "at will" worker, your company can set the condtions of your employment much as it sees fit (absent some form of legally actionable discrimination). That having been said, since you are non-exempt you are most probably paid hourly. If this is the case, you must be paid for all time that you perform your job duties and having to answer after hours phone calls would count.