can an employer make be "on-call"?

Yes, but in some cases the employer must pay the regular and/or overtime rate for the on-call time.  There are lots of ins and outs here, but the general rule is that if the employee is "on-call" but is otherwise free to pursue his non-work activities, he is not required to be paid until he is called in.  However, if the "on-call" status substantially restricts his non-work activities he may be entitled to pay for those on-call hours. As to whether the employer can require the employee to be available or on-call, the answer is generally "yes".