Ozeki .Net SDK for C# SIP WPF softphone development to make/receive VoIP calls. It supports basic telephony functions like make/receive, reject, hang up, hold calls, call transfer, DTMF handling for IVR systems, HD video, extended codec support. The GUI of the Ozeki WPF Softphone sample program has been developed with Microsoft WPF. It is easy to change skins and to deliver crystal clear sound. Customizable with further features & company logo. The Ozeki SIP SDK provides tools for .Net developers to develop a high quality C# WPF softphone. For developing a C# WPF softphone high voice quality is essential and with the help of Ozeki SDK this feature can be easily reached with the extended codec support: 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, etc. Ozeki SIP SDK also works with almost all VoIP service providers (Skype Connect, Gafachi, Callcentric, SureVoIP, VoIPtalk, etc.) due to that it is fully standard compliant. Information can be gathered about the phone line status with the PBX like successful or unsuccessful registration, registered or not registered. The source code can be downloaded freely to use it or modify it. Due to the flexibility of Ozeki SIP SDK, the developed softphone can be further improved with features like: Jitter buffer, Voice Activity Detection, Acoustic Echo Cancellation, Packet Lost Concealment (PLC), Call timer, Customized with Company Logo/Name, Unlimited User License, Full HD video, Multiple lines for multiple concurrent calls, Conferencing, Call history, Quick calling and re-dialing, etc. The sample program handles one telephone line but with Ozeki SIP SDK there is an opportunity for developing and using multiple telephone lines simultaneously.