It's ~$13/year to keep the same phone number, and <= $0.01/minute for all incoming calls and any outgoing calls to North America. Most of the cost comes from outgoing international calls, whose price varies (see https://voip.ms/pricing), but I do disable some of the most expensive destinations.

With paying for ~60 hours of calls for the week it runs, I think it's still usually well under $100, though.