Hi
It is very essential to keep in mind that the person who you call up for the referral of a potential candidate is actually doing you a favour. Hence one must be very polite and ensure that you do not take up too much of their time.
It is best if you have a couple of pre planned questions kept ready so that the moment you call up and they are ready to respond to your queries you do not waste any time.
Immediately after calling up identify yourself and your organization and quickly tell them you are calling in connection with the hiring of the Candidate (give the candidates name) who has given the reference. Also ask if it is a good time to talk or seek a suitable time from that person to call back again.
Once the person you call is ready to answer your queries ask them the questions you want answers to about the candidate under consideration.
The questions could be as under :
How do you know the candidate
How long have you known the person
How would you describe the person
Finally end the call with a warm thanks for sparing time and the reference.
Best Wishes