StarTrinity SoftSwitch 3.1 could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this software available. The following version: 3.1 is the most frequently downloaded one by the program users. We recommend checking the downloaded files with any free antivirus. The program is included in System Utilities. The actual developer of the free software is StarTrinity.com.
From the developer:
The software is a free Windows-based class 5 softswitch with billing and web interface. It can be used for VoIP wholesale, origination, termination, also as a platform for building various VoIP applications: PBX systems, IVR servers, conference servers, SBCs, call centers, auto-dialers, etc. Call procesing in the softswitch is based on CallXML scripts. The softswitch is a continuously-self-tested system with high stability and performance.
You may want to check out more software, such as Billion Softswitch, Billion Phone or Mizu VoIP Server, which might be similar to StarTrinity SoftSwitch.