How do I start phone-to-phone call with webcalldirect(betamax) in vb.net in my program?

Question

Hello. I need to implement the phone-to-phone funcionality in vb.net. I've got the sip-account information from webcalldirect, but have no clue how to initiate a phone-to-phone call? In their clientapp their is such a function, so i thought it must be possible to do the same in my own clientapp.

Thanks in advance