“…Hence, we can leverage the ratio between incoming calls and outgoing calls as a feature for discrimination. Although many features characterize this fact, e.g., IR, HR, SR [5], CI [6], the ratio between outgoing and incoming calls [16], BDR (Bi-Directional Ratio) and IOR [19], we use IOR in this paper. Both BDR and IOR focus on "the number of callees" instead of "the number of outgoing calls" used in IR, HR, SR, and CI.…”