Virtual Private Asterisk
Beave writes "These guys
have posted an interesting article on using
Virtual Private Servers to secure and even run multiple instances of Asterisk on one machine.
The article has in depth instructions on how to get such a beast up and running using hardware or emulated hardware (ztdummy driver). It's like running a small telco in a box!"
There are any number of SIP softphones, but I recommend using a softphone using asterisk's native IAX protocol. Check out the iaxcomm softphone for win32, linux and OSX: iaxclient.sourceforge.net/iaxcomm/index.html