Hello and welcome to ppcgeeks. I'm currently shopping for a new voip provider so I can't comment on SJ-Phone, but usually you can contact your voip provider and they'll provide you details on what information you need to enter in the SIP. Also, I have heard that this windows mobile app (shareware)
http://www.ageet.com/us/products-agephone-mobile.htm supports voice through headset on some HTC devices.