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I install an ubuntu desktop in my company network (windows network) that use SIP on software phone and hard SIP phones.
I use Jitsi that is a good client for SIP and works well with others software phones on the network but fails to decode streams from and to hard SIP phones.
It looks like the main codec for decode RAW data from hard phones is g723 / g729 but I didn't found any resources on it for get it for linux/ubuntu
1You are using the SIP Server to make transcoding also. It's a good option for a few simultaneous calls but it won't scale if your number of Linux clients increase. – jcbermu – 2015-05-25T08:50:54.573