I've set up freespeech with my Linux machine.

My setup is as follows:
Broadband: Eircom
Router: 3Com Office Connected Integrated ADSL Modem/Wi-Fi/Router/Firewall/Hub
Laptop: Dell Latitute D610 Connected using Wi-Fi. The laptop runs Ubuntu Dapper Drake.

I've configured the router to passthrough port 5060 for SIP.

The Soft Phone I am using is Ekiga (formerly GnomeMeetting)

The following settings are in place for the account.
Account Name (whatever you want)
Registrar : freespeech.ie
user : (your freespeech 076 number)
password: (your freespeech password)

Also set up in preferences:
Protocol: SIP Settings..Misc Settings..Outbound proxy: freespeech.ie

Audio Codecs: In Ubuntu Dapper, iLBC and PCMA are available, so it all works fine.
I've tesed this using a call from my mobile and everything is working.

Looks like a very nice client (linux folks seem to be spoilt for choice with SIP clients). Good post - spread the word.