You need another service for the OBi to send the call out on. A POTS line can only support one call at a time, so an incoming POTS call cannot be forwarded over that. Therefore yes, you must have service from a VoIP provider. Google Voice is one and being 'free' has attractions but there are many other Internet Telephony Service Providers whose additional features may warrant their charges which are generally very modest when compared to a typical landline provider.