OBi 1-5:
Physical Interfaces -> PHONE Port -> DigitMap:
([1-9]x?*(Mpli)|[1-9]|[1-9][0-9]|911|**0|***|#|**1(Msp1)|**2(Msp2)|**8(Mli)|**9(Mpp)|<2*>(Mpli))
Physical Interfaces -> PHONE Port -> PrimaryLine : SP1 Service
User Settings -> Speed Dials -> 2 : PP(ob200000006)
where 200000006 is the OBiTALK number of OBi 6.
OBi 6:
Voice Services -> OBiTALK Service -> InboundCallRoute:
{(200000001|200000002|200000003|200000004|200000005)>(Msp1):sp1},{ph}
where 200000001|200000002|200000003|200000004|200000005 are the OBiTALK numbers of OBi's 1-5.
Voice Services -> OBiTALK Service -> MaxSessions : 4
NOTE: Four simulataneous ougoing calls is the limit because the OBi is limited to four bridges. Four simultaneous outgoing calls are possible if you configure two Google Voice accounts (SP1 + SP2), set PrimaryLine to Trunk Group 1, and change 'sp1' in the InboundCallRoute above (2 places) to 'tg1'.