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http://skype.com - http://skype.com
Peer to peer voice service. Users may call landlines and cellphones for a fee; users may call each other for free. Source code is not available. |
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Asterisk - http://asterisk.org
Open Source telephony switching and private branch exchange (PBX) daemon. Supported signalling protocols: H.323, SIP, MGCP, SCCP (Cisco Skinny). |
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http://www.ekiga.org/ - http://www.ekiga.org/
Videoconferencing application to make audio and video calls to remote users. Supports both SIP and H.323. |
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OpenH323 Gatekeeper - http://www.gnugk.org/
Free H.323 gatekeeper (GPL) based on OpenH323. |
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Linphone - http://www.linphone.org/
SIP based web phone. Supported audio codecs are G711, LPC10-15, GSM, and SPEEX. |