When, during a call, your client says he does not have time right now you do not have to worry. You can easily reschedule a call for any time and date. It's very simple. To do that you need to click "Schedule next contact" button at the orange bar located on the bottom.
Once you click on it you will be able to choose a date and a time. The system will show you a calendar and proposed hours (every 15 mins), although you will be able to enter custom hour. Additionally you can add a note why the call has been rescheduled, so you will know what to say next time the number is called.