Five9 is a great partner to work with. They provide really good pricing models, and like other companies don't tie you up in tiers, and contracts and agreements. It's really a low friction, no frills way of providing great VOIP services to customers. Further, I'd say their tech support staff is phenomenal. They are able to help you and walk you through whatever it is you're trying to do at any time or day. 

They're doing a great job and Five9 is my go to recommended choice when asked for recommendations from our industry.