By far, the essential aspect when choosing a phone provider is the quality of the service. Many people look at the price first, but doing this may get you dropped calls or scratchy sound, which is a waste of money and can hurt your business.
Look to a provider that offers the highest level of call quality you can get. A business needs dependable service with no dropped calls and with a clear voice on both ends of the call. Remember, though, that call quality depends a good deal on your internet connection, so having the highest speed you can get with the most bandwidth available is a huge plus.