X-Lite

X-lite is a soft-SIP-phone. It is the free (a little limited) version of Counterpath's Eyebeam.

Tip's 'n Tricks

• When in conversation, press CTRL-F9 to see
  
  some debug-information about the network-speed and codecs used.
• If the connection fails after a few (tens of) seconds, check if both sides use the same codecs and disable all other codecs
• If the connection still fails after a period of time, try un-checking Send SIP keep-alives on the advanced section of your SIP account.
• If the other party cannot (or can hardly) hear you, check the recording-mixer settings of the mic to see if some other program may have touched the mixer-settings.
• eyeBeam 1.5 FAQ

Things related to bandwidth usage

See the Controlling Bandwidth section.

X-lite 2.0 (linux)

Documentation very limited on this linux version of the softphone. Menu entries that need to be set:

Enabled: Yes 
Username: 200 (my extension)
Authorization User: 200 (my extension again)
Password: abc123 (my extension password)
DomainRealm: 129.125.21.240 (trixbox IP address )
SIP Proxy: 129.125.21.240 (trixbox IP address )