Hola dejo esto para estudiar mas adelante :
https://www.microsip.org/
https://www.ekiga.org/
http://www.linphone.org/
https://jitsi.org/
http://www.phonerlite.de/index_en.htm
https://www.zoiper.com/en/voip-softphone/download/classic#providers
http://icanblink.com
