Localphone for outgoing at $0.005/minute or cheaper with one of their subscription bundles.
You can rent an incoming phone number from them ($1/month), maybe port your number to them ($?/month), or point a free IPKall incoming number at your SIP ID with them.
If the minutes are mostly incoming, Anveo with a $4/month business unlimited number gives you all kinds of nice features and has the cheapest E911 service I know of ($0.80/month).
For business use you might want to keep it simple and reliable... Localphone or Anveo with an incoming number and maybe you'll want to forward your google voice number to that number.
Or set up your localphone account now, add and verify your google voice number. No cost. Sign up for an IPKall number, point it at your localphone SIP ID. Call the IPKall number to make sure it is working. Add the IPKall number to google voice and forward to it. Now your incoming goes GV->IPKall->Localphone->OBi, no cost. Outgoing goes OBi->GV, no cost. Later you can add funds to your Localphone account and do outgoing via OBi->Localphone at low cost.