Set it up then leave it
Our service has been reliable, I don't think it has ever gone out. There is a lot of call routing control, and many À la carte options which helps us keep costs down. We offer receptionist services to our customers with relatively high call volumes. Fonality's system has been perfect for us.
We did have some EXTREMELY frustrating difficulties when getting setup with Fonality, however it was very nice that I could call support at any time. I made my frustrations known to customer support and they got my issues resolved. We've recommend Fonality to others who seemed to have a better time setting up.
I've experienced a fair amount of "misunderstandings" both with sales and support. When it comes to support I've taken a "if it's not broken, don't fix it" approach. For sales, I found a reliable sales rep (Justin, extension 6436) who understands what we need and delivers it to us.
We've worked with RingCentral, Grasshopper, Phone.com, Google voice, Skype, and others. Overall, I recommend Fonality.