Dialpad - Switch is a cloud-based phone system built for Google Apps users.
3CX - Hosted PBX with CRM integration and mobile apps
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
CallHub - Predictive Call Center, Voice and Text Broadcasting app
Asterisk - An open source telephony switching and private branch exchange service for Linux.
Digium - Digium offers IP phones, business phone systems, such as Switchvox IP PBX, and custom communications solutions for Asterisk.