I would also second RingCentral, however you're looking for a VoIP therefore RingCentral wouldn't nessecarily suite your needs! Although I have no experience with this company, take a look at ReignMaker
try teliax and junction network they are both good
Core Incorporated. An Open Source Telephony Consulting Company. - Call Center Technology Consulting. VoIP wholesale, Hosted and Premise Based VICIDIAL Open Source Predictive Dialer, Open Source Telephony Consulting, Call Center Technology Consulting.
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Pay as you go Voice Blast/Broadcast service! Only pay for the VoIP minutes you consumed.