The invention discloses a realization method and a system for callback; wherein the method comprises: after a first terminal triggers Unstructured Supplementary Services Data service (USSD), a core network of the roaming domain of the first terminal routes the USSD request message sent by the first terminal to a service control point of the home domain through a home position register of the homedomain of the first terminal and then triggers the callback service of the service control point; wherein the USSD request message carries the number of a second terminal; the service control point sends scheduled message to the service control point to inform a service switching point to call; the service switching point calls the first terminal and the second terminal to realize the conversation of the first terminal and the second terminal. The adoption of the invention has the advantages of completing the call under service control when the user roams, ensuring more reasonable charging, and avoiding the problems of unreasonable charging and undetermined rate selection mode of the related technology.