What are some alternatives?
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
3CX - Hosted PBX with CRM integration and mobile apps
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Vonage Business - Business phone system.
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
Digium - Digium offers IP phones, business phone systems, such as Switchvox IP PBX, and custom communications solutions for Asterisk.