Google Voice is not intended to be a business telephone solution. It's intended for individual consumer use. You've already realized a couple of its limitations. Yes, you can provision the same Google Voice phone number on more than one OBi IP phone, but it does not have the typical call-handling functions that you'd get with a business phone service provider.
The present-day solution for SMB telephony is to use a cloud-based SIP VoIP service provider. Essentially a virtual PBX, you can add, change or delete extensions, use voicemail, set up hunt groups or simultaneous ringing, etc.
If you're tech-savvy, Callcentric or
voip.ms are less expensive, but need some DIY configuration skills. Dialpad and Ringcentral are examples of more advanced services with tech support and more features. All will work with your OBi IP phones.