the present invention relates to a method for checking a data exchange between participants in a call established on a communication medium in compliance with a protocol . in order to identify the various stages in a call process conforming to the protocol and to break said protocol once the checking procedure has started , the invention suggests that the data exchange be captured by - means of a protocol monitor connected to a communication medium and based on the finite state automaton principle . the test automaton contains the same states and state variables as the protocol automaton designed to define the communication protocol , apart from the fact that the test automaton is allocated a plurality of values within the same range of corresponding state variable values in the protocol automaton . the transitions in the protocol automaton as observed on the communication medium are simulated for investigation purposes in the test automaton and , by a process of logical elimination , a state is shaped in the test automaton which matches the relevant state in the protocol automaton .