Asterisk is a open source telephony switch and a private branch exchange (PBX) software based on Linux. It is one of the most widely used communication system.
ViciDial is a complete opensource Call center suite that works with Asterisk. It is being used by many large Call Center around the world. Comparing to many high level branded call center suite, ViceDial has more inherited features then others.
A2Billing is a calling card & billing platform based on Asterisk PBX. It is a professional and powerful billing solution, post paid and pre-paid oriented, real time call rating, billing, auditing, and reports.
FreePBX is a open source is a web based open source graphical user interface (GUI) that manages Asterisk and Voice Over IP (VOIP) and telephony server. FreePBX can be installed as standalone software, that includes the operating system, the Asterisk PBX, FreePBX, and assorted dependencies.