One common usage is like this:
Voice-services->GW and Trunk Groups-> TrunkGroup1-> Trunk List = sp1, sp2 (this is already the default)
and also make the Digit Map = Msp (same as SP1 and SP2)
Phone Primary Line = Trunk Group 1
AA Primary Line = Trunk Group 1
If you are using GV, you can only make one call at a time. You can configure both SP1 and SP2 with the same Gmail acct, and now you can make 2 calls (press "flash" to put the 1st call on hold, and flash again to conference). Hence 3 way calling with the OBi/GV is possible to do.