I've successfully called a real phone in the UK from my Zaurus in Japan.
I've only used it with the sipphone.com service, here are my tips to get it working.
-  There seems to be a problem with the sipphone.com service (not kphone), I contacted their technical support and they gave me a workaround. Now by dialing 099 + country + area + number I am able to successfully make voice calls to real telephones. You should just be able to dial 00+country+area+number but this doesn't work.
 I have found that the stereo headphones I have make a perfectly acceptable mic and earpiece. Just use the right ear as the earpiece and the left ear as the mic. The mic quality is not very good, but usable for phone calls. I have also used a Japanese mobile phone "hands-free" set. In this case you also need a 2.5mm->3.5mm jackplug adapter. The audio quality from this mic much louder and clearer than using headphones for a mic. A final idea for an audio setup is to get an adapter used to connect 2 mono mics into a stereo jack, you can plug a mic into one input and an earpiece into the other. I haven't tried this myself, but I think it should work.
 kphone 4.04 works better than kphone-pi -0.97. Both are able to make calls, but kphone-pi cannot hang up. The application just stops responding to user input once the connection has been made. To terminate the call with kphone-pi you must kill the kppi process, and get the other party to hang up their end. Kphone 4.04 works correctly, just click the Hangup button to disconnect the call.
 Both kphone and kphone-pi have problems registering with the sipphone.com service. You need to be registered with the service (the client normally registers you with sipphone.com when it starts up) before you can successfully make a call. The only way I have found to get registered is to start a second client when the first client is still running. For some reason the second client pops up a dialog box asking you for your password and then successfully registers, the first client doesn't do this and eventually times out trying to register. This is a bit messy, hopefully someone can figure out an elegant solution.
The calls are pretty cheap, and the audio quality is very good, however there is often a delay on the line of about one second and this is quite difficult to get used to (perhaps this delay will not be so bad when calling from the US). Also, the sipphone.com service offers 5 minutes of calls free each day to various countries, very nice, but I have yet to be able to make a free call with them with any client (including their own in WindowsXP).. this may of course change at any time, or be peculiar to my setup, since things in the world of SIP phoning seem pretty volatile.