Do you think it is OK for a supervisor to fire an employee for getting a call from another company they applied for? Is it standard business practice to call someone's current employer to ask about them?

No, I don't think it's ok to fire an employee for simply looking at other job opportunities. However, I think it is common courtesy to at least let your boss know that you are applying elsewhere. Otherwise, it looks very sneaky to your boss. Just tell him that you are interested in what other employers have to offer...