This is a tough category because all telephone service companies have complaints on public forums. Remember that people who comment on these forums are more likely to be unhappy with the service, while happy people don't frequently take to publicly announcing their happiness with a product. Acknowledging that all the companies we looked at have some complaints, the winner for best service is Vonage, one of the first phone companies to offer VoIP for business.
All of these questions are pertinent to their call, and it’s important that you answer any that is relevant to your specific situation. Make sure not to drag on too long going through all of the info.

Does the service provider have their own network? By putting office connectivity, phone, and servers on one network you'll enjoy better performance and managing your data and communications will be a lot easier. Are the data centers state-of-the-art and easily accessible?
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.