4PSA VoipNow is the leading Linux IP PBX software for Service Providers and enterprise. Easy to use and deploy, 4PSA VoipNow softswitch helps you increase productivity and reduce phone bills. 4PSA Express free edition offers standard PBX features and is limited to ten phone extensions, while Professional (commercial) edition offers in addition more users, prepaid and postpaid billing plans, IVR s,calling card, callback, conferences and call queues.4PSA VoipNow work with x86 and IBM PowerPC based servers. The system can sustain several hundred extensions even on an entry level server. For more details about the software installation, check:http://www.4psa.com/docs/voipnow/voipnow_commandline_installation.html