All outgoing calls are billed per minute regardless of plan. Rates vary based on where you're calling -
https://voip.ms/rates.php has a list.
I doubt you'd find something in the same price range (<$10/month) for an unlimited business line. You'd be more likely to find things in the range of $20-30/month/concurrent call for an unlimited business line. Vitelity (Virtual PRI), SIP.us, SchmoozeCom SIP Station, etc. all have plans that fit my mentioned price range. There are definitely others, but those are the first that came out of my head.
Edit: The flat rate VoIPms plans are for residential use only. Business use violates the ToS. This restriction is mentioned when you order each DID.
Restricted to residential use, up to 3500 minutes per month
I assure you if they detect business-level calling on a line they will back-charge you for the difference.[/QUOTE]