Yes, an employer may extend an employee's shifts. Absent a binding employment contract or collective bargaining agreement (union contract) that limits the practice, the employer can extend the shift in accord with its business needs. The employee must be paid for all time worked.
The issue here may not be that the employer is trying to avoid paying the on-call minimum, but that it is trying to provide good customer service by scheduling as many service calls as possible within the standard work day so that the customer does not have to pay a premium for emergency / after hours service. The effect may be similar, but the employer's focus may be on customer relations and avoiding losing customers to its competitors.

