GV is a VoIP service. You can only have 2 VoIP services set in the OBi devices, so here are your options:
-2 GV accounts, or
-2 SIP accounts, or
-1 GV account and 1 SIP account.
If you want more services available you will either need:
-a hosted pbx service, or
-your own switch, like asterisk, or
-more OBi devices to access those VoIP services.
Dave