Ozeki VoIP SIP .NET SDK to develop Flash chat program for text/audio/video chat between more Flash clients. The sample program uses Flash as the client-side technology. Its GUI has 3 panels to ensure basic telephone functions and 2 pop-up windows. It allows to build the chat server, handle clients from the server side, server support for text message sending, camera handling, server side support for voice calls, text message sending to selected clients, audio chat function, video phoning from the chat client, sending response to the remote client's media request. The Ozeki SIP SDK supports simultaneous VoIP phone calls. The chat application is a client-server solution as the client programs cannot communicate directly with each other by sending audio and video data. The server is used for establishing the connection between the clients and for transferring the audio and video streams to and from the clients. The server of the sample program is a console application written in C#. As the server itself is only a connection provider and supporter, it does not need a GUI. It is written in Visual Studio 2010 and uses the extraordinary background support of Ozeki VoIP SIP SDK so basically, the server only contains some method calls and setting for the communication. Ozeki VoIP SIP SDK has been developed to ensure excellent sound quality. For this purpose various codecs are supported: G.711 aLaw/uLaw, G.729, iLBC, GSM, G.722, SPEEX, SPEEX-WB. To make/receive VoIP phone calls, the WPF softphone is compatible with almost all PBX systems like Asterisk PBX, Cisco UCM, 3CX Phone System, Kamailio PBX. Ozeki SIP SDK also works with almost all VoIP service providers (Skype Connect, Gafachi, Callcentric, SureVoIP, VoIPtalk).