In this paper we present our original approach to the model-based statistical usage testing of a class of communication protocol implementations that are based on the State design pattern and Java programming environment augmented with the class FSMSystem. The approach is based on the working environment that has been proven on a number of real-world projects. The working environment is created with the following set of tools: generic modeling environment with the operational profile model paradigm registered to it, operational profile model interpreter, generic test case generator, test case compiler, and the unit testing framework JUnit extended with the class TestBed that acts as both test driver and stub thus providing the complete test harness. In the paper we present the methodology of the model-based statistical usage testing of a class of communication protocol implementations, the tools that support this methodology, and the case study - the model based statistical usage testing of SIP INVITE client transaction, a part of the SIP protocol stack.