Here is an interview with Nextiva's CEO. "We now have 6 geographically redundant datacenters across the United States and serve more than 120,000 business customers using one or more of our hosted VoIP, SIP Trunking, call center or other unified communication solutions." 120K billing customers but not necessarily HPBX. - See more HERE.
In a video, Diane Myers of Infonetics explains why 8x8 is the #1 Cloud Unified Communications Provider. See it at YouTube.