A method of generating a set of test sequences for evaluating interoperability of a number of interconnected communication systems with respect to a desired mode of communication between a first end user and a second end user. A number of operational states that are required of the communication systems to implement the desired mode of operation between the two end users is determined, wherein each state pertains to a first operation of a first gateway system associated with the first end user and a corresponding second operation of a second gateway system associated with the second end user. The interconnected communication systems are tested by causing the systems to perform specified transitions between pairs of at least some of the operational states.