Google Voice is a telephony service that provides call forwarding and voicemail services, voice and text messaging, as well as U.S. and international call ter…

Asterisk is a open source communications project that lets you create telephony apps for IP PBXs, VoIP Gateways, etc.

Twilio is a cloud communications platform as a service (PaaS).

KeepCalling is an online calling application.

Bitrix24 is a free (for small businesses) social enterprise platform.

TeamWox is a business collaboration software.

Plivo (Plēvō) is a cloud-based communication platform as a service (CPaaS), CPaaS fuses communications apps with business workflows.

Voximplant helps to embed audio and video communication into your apps.