You can potentially fork calls at the provider, in the OBi, or both.
Vitelity has no forking ability, except for simultaneously ringing two SIP devices.
Callcentric allows ringing up to three destinations simultaneously, which can be PSTN, your SIP device, or a SIP URI. You could use a second (free) account to expand beyond three.
Anveo allows ringing up to four destinations;
VoIP.ms allows at least four and may have no limit.
With all of the above, calls forwarded to PSTN numbers are billed as normal outgoing calls. Of course, if you forward in the OBi, there would be no charge if you used GV for the outbound leg, but then you would not be able to pass the original caller ID to the destination.
Some "unlimited" providers, including VOIPo and Phonepower, permit forwarding to multiple PSTN numbers without a per-minute charge. (Those calls do count against your cap, but with either of the two mentioned, it's 5000 minutes, so you are very unlikely to run over.)