I don't know if you have to have a sip phone provider to test it or not. I think there is a way but I have never tried it. Rather I have a diamondcard.us account which allows me to call anyone's land line or cell phone on linphone, whether they have linphone installed or not. I am pretty sure, without knowing, that linphone's video feature (and voice) may work without a sip provider, provided the other party has linphone installed on his or her computer.
I am trying to get my sweetheart to install linphone on her mac and will let you know how that goes.
This whole sip business is a bit new to me. However, I use Twinkle now constantly as, using diamondcard.us, it can call any phone anywhere in the world for just a little more than nothing. Linphone will do the same as Twinkle, plus the video (but, I believe, minus twinkle's handy conference call feature). I am hoping that linphone 3.5.2 has better sound quality than linphone 3.4.3, which I was using.