Whenever I schedule a phone meeting, I attempt to make them 15 minutes.
It tells the customer that I’m only going to take up a small portion of their time. Even if I let the conversation run long, they don’t see it as a full half hour commitment. Specifically because of the positioning of the appointment.
Think in terms of when something is scheduled. The less time committed, the more likely they are to commit. Because they don’t rationalize the overall scope of how much time will be used, compared to the time that they believe will be taken.
It’s a psychological ploy, but it works in sales. Specifically because no one will block out the 15 minutes afterward for another meeting, and if your phone conversation runs over, they justify it as a valuable conversation that just extended longer than intended.