I recently did a sound quality test between a handful of providers. The calls to my Callcentric DIDs sounded noticeably better than the other providers. I even pointed my IPKall DID to my Callcentric SIP address and it sounded better! The calls had kind of a 'warm' sound to them that was lacking with the other providers.
Sure, it would be nice if they had features like sub-accounts and such. Fortunately, it is very easy and cheap to set up an asterisk box (thank you Raspberry Pi!) between your phone and Callcentric. Then you can add all of the features that you want!
Also, support is great. I was receiving responses with a recent support ticket within 30 minutes (or less) AND I believe it was close to midnight on the east cost!
united state
Contract price not specified. 
"'warm' sound quality & great reliability. Useful free products offered."
"Some providers have more features"
"great provider with outstanding voice quality"
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