3CX - Hosted PBX with CRM integration and mobile apps
Grasshopper - Grasshopper enables entrepreneurs to build a virtual office that connects callers to employees, wherever they are.
CallHub - Predictive Call Center, Voice and Text Broadcasting app
CallMultiplier - CallMultiplier is an all in one voice broadcasting and text messaging service that allows you to send automated calls and text.
Asterisk - An open source telephony switching and private branch exchange service for Linux.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies