I believe that you should base your rates for telephone interpreting on the concepts of a minimum and the expected burden on your time. No matter how short the telephone time is expected to be, you should consider the time required to accept the assignment, wait for the call, perform the assignment, invoice the assignment, process the payment, and possible charges to your phone. You should also consider inconvenience to your daily schedule. Taking these factors into consideration, I could not expect a charge of less than one full hour at your normal interpretation rate. If the scheduled call falls outside your normal business hours (like 3:00 am local), a hefty premium is very reasonable. It is also my experience that clients requesting telephone interpretation, if not already known or easily identifiable as a responsible party, probably deserve more than usual scrutiny.
Good luck, but be careful
Thanks Marc. I guess like with all translation work one has to be careful, especially as you never really see the people who are employing you! Thanks again.