Devices that enable compatibility can't be found easily. I have met an sdk with several sample programs (a sample program for building a softphone can be viewed here: www.voip-sip-sdk.com/p_179-windows-forms-softphone-vb-net-voip.html )which is written in VB.Net and therefore insures flexibility including pbxes and further developments. Hopefully you treat this message as a useful advice which aims to help you.