In the old days, each line required a physical copper pair between you and the central office. They were expensive. With VoIP, you can be free of the ‘lines’ concept. Pay a small monthly fee per number, plus a per-minute charge for calls in and out. Customers never get a busy signal. You never have to ‘wait for a line’ to call out. You don’t need to plan for and manage seasonal variations, promotions or growth. If you like this pricing model, my first choice would be Flowroute. Other first-class (IMHO) providers offering this are Vitelity, Callcentric, Twilio. A cut below, look at Anveo, VoIP.ms, Plivo. I’m also familiar with the down-and-dirty, but won’t discuss them unless cost is your primary concern. On Flowroute, for one toll-free and one geographic number, expect to pay ~$4/month + $0.015/min. (other than toll-free incoming) + $0.025/min. (incoming toll-free). This includes an estimate of per-minute rounding, USF charges, CNAM lookups, SOMOS dips and E911.
However, pricing based on channels (‘lines’) is also available. Some users prefer a predictable bill, even if it’s somewhat higher. Also, some providers promote this model, because it makes for easier comparison with traditional carriers. Unmetered incoming (on a local number) with three or four channels is offered by Vitelity, Callcentric, Anveo and others. With these plans, outgoing calls are still billed per-minute. There is usually a usage cap (Vitelity’s is 4000 minutes) or ToS restrictions against telemarketing, etc. Although these are an excellent value for small businesses, beware that if growth requires additional channels, you may need to go to a much more expensive plan (or change providers).
Several providers offer a flat-rate plan for both incoming and outgoing. Although these are IMO generally poor values, SIPStation is an exception because they include numerous features, some of which are unavailable elsewhere. See https://wiki.freepbx.org/display/ST/SIPStation+Key+Features+with+FreePBX+and+PBXact . There was a recent discussion of this on Broadband Reports that became quite heated; see https://www.dslreports.com/forum/r31991764-Nerd-Vittles-hit-piece-on-Unlimited-SIP-Trunks . The last post is my attempt at presenting a balanced viewpoint. It was apparently successful, as it was upvoted by both adversaries.
My primary provider is AnveoDirect. Incoming numbers are $0.19/mo. + $0.004/min; carrier is Bandwidth.com. Over the past four years, it’s been flawless except for one outage caused by a fiber cut in my Reno, NV rate center. Reno is pretty small with just one ‘carrier hotel’; any other provider would have been similarly affected. Outgoing uses their ‘smart’ routing with a limited set of carriers, averaging ~$0.005/min. for my calling pattern. Failover (less than 1% of calls) goes to Flowroute. To toll-free numbers (free on Flowroute), the order is reversed. E911 is also with Flowroute.
We may be able to give better advice. Post your approximate usage, cost expectations, countries called, etc.