CallHub - Predictive Call Center, Voice and Text Broadcasting app
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
CallHippo - Easy to use Phone System with World-Class Support
Melrose Labs CloudPBX - Advanced Hosted Cloud PBX for Your Team.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).