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.
Before you can start considering a brand, you need to figure out exactly how you want your business to use a phone system. Look at your existing phone system and decide whether you're going to simply keep all of it and bolt some VoIP functionality on top, retain only part of it, or replace the whole thing. Frequently, a total replacement isn't in the cards if only because some parts of your existing phone system can't be easily changed over to softphones or even desktop VoIP handsets.

Hi! I told you never to call me at this number. But since you did, I left the knife on your back porch…Check to see I got all the blood off. Don’t forget to leave a message, and we’ll talk about the…plan.
Affordability -- What are the up-front, monthly, and maintenance costs, and are these sustainable? Are you locked in to purchasing more hardware or software options?