This is exactly what I am worried about now.
I tried doing a reference check for my wife who left the organisation years ago with the intention of knowing if a verification can be done by phone or not. I pretended to be the HR official of another Hospital.
On the first attempt: The call landed to the receptionist who tried to transfer the call to the HR dept, where the phone rang endlessly and no one answered.
Second attempt: This time, I called after an hour and spoke to a different receptionist, who was talking to someone else and me at the same time. After stating the purpose of the call thrice, she transferred the call to Mr. Fernandes of the HR who told me that they don't have data of employees employed 3 years ago. He stated it is hospital policy. (I was sweating when he told me this, come on man, why would you have a company policy that states something like this..)
Third attempt: I called and the receptionist who answered my call the first time transferred to another person who asked for the employee code and told me the period of employment.
I didn't dare to do the reference check for the second reference point.
Moral of the story:
If your VO has a bad day, I don't think he will attempt to call 3 times. If you are providing references of big organisations, this can happen.