1) sudo apt-get install asterisk

2) Add the following to /etc/asterisk/sip.conf

[alice]
type=friend
username=alice
host=dynamic
context=intern
canreinvite=no
nat=1
context=from-sip

[bob]
type=friend
username=bob
host=dynamic
context=intern
canreinvite=no
nat=1
context=from-sip

3) Add the following to /etc/asterisk/extensions.conf

[from-sip]
exten => alice,1,Dial(SIP/alice,20)
exten => bob,1,Dial(SIP/bob,20)
exten => test,1,Dial(SIP/613@fwd.pulver.com,20,rt)
exten => test2,1,Dial(IAX2/guest@misery.digium.com/s@default)

4) Follow http://iki.fi/lindi/sip/kphone.txt to install a SIP client
   and test your setup.