An apparatus and method for detecting potentially-improper call behavior (e.g., SPIT, etc.) are disclosed. The illustrative embodiment of the present invention is based on finite-state machines (FSMs) that represent the legal states and state transitions of a communications protocol at a node during a Voice over Internet Protocol (VoIP) call. In accordance with the illustrative embodiment, a library of FSM execution profiles associated with improper call behavior is maintained. When there is a match between the behavior of a finite-state machine during a call and an execution profile in the library, an alert is generated.