In general, VOIP providers offer various tiers of services that are designed to meet the needs of businesses of all sizes. The main feature to look for is the number of users included with each plan to ensure that you get enough lines to ensure all of your employees are connected. Most of the time, prices are quoted on a per-user basis with a minimum and a maximum number of users allowed for each plan.
You're essentially sharing a large PBX with that provider's other customers, but because these companies use multi-tenant segmentation, your PBX will appear dedicated to you. This engine will take care of routing calls on your VoIP network and out to others as well.

The problem there is that VoIP traffic is much more sensitive to network bumps and potholes than most general office traffic. That translates to garbled conversations, difficulty connecting over Wi-Fi, or (worst case) dropped and lost calls. If your business is small and your network is essentially contained in one or two wireless routers, then your configuration and testing headaches might be fairly easy (though still there). For medium and larger networks, these tasks can be complex and time-consuming, which translates into added cost in terms of man-hours.
Vendors like VirtualPBX sell hosted systems to offer businesses a simple method of creating voice networks. Businesses purchase subscriptions from a vendor and connect their IP phones to that vendor’s voice network.