Ask HN: What's the best VoiP service for startups?

I work at a startup with five full-time customer service team members, and 10 other people with phones in our office.

We all use the same VoiP provider, Vonage/Vocalocity, but the service is terrible (calls drop, static, etc) consistently. We've had our IT team configure the settings, upgraded our Internet to a T1, but believe it's our VoiP provider.

I'm looking to see if anyone can recommend another VoiP provider they've had experience with 5+ phones connected and heavy usage.

Thanks in advance.