This paper focuses on Iran next generation network (NGN) Pilot, the purpose of which is to achieve appropriate knowledge for a desired move through current telecommunication network of Iran to NGN. On the base of this purpose, obtaining a proper knowledge about capabilities and weaknesses of vendors' NGN equipments is necessary as well as verifying the functionalities of these equipments in a multi-vendor environment. Accordingly, a test bed has been designed with the capability of executing various tests scenarios related to NGN and processing the results. At these stage two types of tests, i.e. Functional tests and Interoperability tests have been done. Since Functional tests are performed in a single-vendor environment; the proper interoperation among multi-vendor equipments can not be guaranteed. Consequently, other types of tests called Interoperability tests have been designed. These tests are performed in a multi-vendor environment and classified into two groups which process interoperation of two Call Servers or a Call Server and Gateway from different vendors, respectively. The following paper represents the results obtained from the mentioned tests as well as the precise assessment of them.