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.
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?

Hi, we aren’t in at the moment, if you are trying to sell us something please start speaking now and hang up at the beep, everyone else start speaking at the beep and hang up when you’ve finished.
Once you finish your voicemail message, you can end on a note of gratitude and express thanks or that you realize the recipient of the message us busy, so you’d appreciate a return call. Whichever way you choose, make certain that the tone of your message is upbeat. A business voicemail greeting sees each call as their chance to engage with their clients or vendors. Hello, you’ve reached the office of Lydecker and Lydecker. We are currently unavailable to answer your call. However, your call is important to us, so please leave your name, phone number or email address, and a brief message and we’ll get back to you as soon as possible. Thank you for your interest in Lydecker and Lydecker Law Offices. Professional tone Provide the caller with specific information Request the caller to leave specific information Extend gratitude for their call Professional Voicemail Greetings