Freeware: h323

TERAVoice Server 2004TERAVoice is a high-performance telephony platform for Windows that can implement any type of telephony application like voicemail, automated attendants, voice menus, waiting queues for call centers, H.323 to PSTN gateway and custom IVR applications.

Teleon Skype Gateway for Voice Modem 2.03Teleon Skype Gateway for voice modem is a VoIP gateway driver for common voice modem card. Calls can be forwarded from PSTN to Skype remote user and vice versa, in fix transfer mode or free dial mode.

GNU Gatekeeper (GnuGk) 2.2.6A free H.323 gatekeeper to build VoIP networks, route calls and do authentication and accounting.