PCBest SIP SDK is one of most widely used tool to develop SIP applications. It is simple, well designed and orginized, fexible and reliable. It will save you tons of time to develop a mature voip application.
VTapi ActiveX Control you get everything you need for quality communications using Microsoft Visual Basic, Visual C++, or any other environment capable of hosting ActiveX components. Create IVR systems, answering machines, detect caller ID and more!