Two of the many options of using DCalling are callback and callthrough.
Generally speaking, callthrough works like this: you dial a local or toll-free dial-in-number to connect with the service and enter the number you wish to call. By now, callthrough services have become even more user-friendly. With DCalling, for example, you can assign speed-dials for as many numbers as you want. Instead of entering the entire destination number, it suffices to enter the speed-dial after you have connected to the dial-in-number. You can also make callthrough calls using the DNumbers. In this case, you just dial the dial-in-number followed by the speed-dial, and you will be connected to your destination number via DCalling right away.
By the way, classic callthrough is also possible with common calling cards, of course. However, user-friendly features like speed-dials or DNumbers are usually not applicable to calling cards.