I've been using Vumber for more than a decade for my company and have been completely satisfied with the product and customer service, and have recommended my clients to use the service as well
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.
Grasshopper - Grasshopper enables entrepreneurs to build a virtual office that connects callers to employees, wherever they are.
Mitel - Business Communications and Collaboration
JustCall - The Cloud Phone System of choice for modern sales and support teams. We help end clueless conversations by adding context to every call.