Asterisk - An open source telephony switching and private branch exchange service for Linux.

Pony Express HQ - Pony Express HQ is an SMS text message marketing for small businesses, real estates, clubs, communities, dispensaries or non-profits. Send and receive text messages and images online with the large group of customers. Group texting app no reply all.

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.

EZ Texting - EZ Texting provides SMS marketing solutions to businesses of all sizes.

freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).

Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.