This Master’s thesis deals in detail with the SIP protocol – a method of communication between two entities, various types of transmitted messages and their content. Few SIP libraries are introduced and two of them are used for development of an application for audio/video conference-calls. Compilation of OPAL and PTlib libraries for Windows 7 Professional (64bit) is described, and problems resulting from lack of information provided by authors. New improved “how to build” is presented. At the end, paper focuses on several ways of development of mentioned application.