Voiceably - Voiceably is the provider of a local phone number to establish an independent cloud call center or business phone system.
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
Line2 - Line2 adds a second phone number or complete cloud business phone system to smartphones & computers that's robust yet affordable. Buy online or get a custom quote.
Asterisk - An open source telephony switching and private branch exchange service for Linux.