VOIP H323 Phone SDK - Version 1.5 VOIP H323 DLL Soft Phone SDK brings H.323 protocol support for ActiveX. With this SDK one can create a simple program to connect and start speaking with anyone with a direct IP Address or use the Gateway or the Gatekeeper to connect to /or from PSTN lines. Now VOIP open source projects have made it easier to implement a customized VOIP Solution eliminating costly PSTN lines.