i would like to know the real answer to that also. Because this is how I see it, unless I am missing something.
Each Google Voice account can support 2 sessions and I imagine the SIP providers will differ but, for the sake of this queston, lets say they can also support 2.
But since there is only phone port, I think the max you can do is 3 way calling, which would be 2 right.
Once you are on a line you can flash to another session on that line and place a call. But flashing again will return you to the first session.
Any way to get more than 2 at a time?