I've only just got a 6120 in the UK. I've debranded it and installed a generic firmware on it..
I entered my SIP settings for my Voipcheap account and selected the always on option from the menu.
My phone connects to my voipcheap account. If someone dials my voip in number they get a ringing tone though nothing happens on my phone. Also there is no option to select an "internet call" when trying to dial a number.
It does make me wonder what was the point of putting the SIP option in the menu in the first place, if there is no way I can see to get it to work!