install asterisk

aptitude install asterisk

edit /etc/asterisk/sip.conf

[general]
context = default
bindport = 5060
bindaddr = 0.0.0.0
tcpbindaddr = 0.0.0.0
tcpenable = yes

[1001]
type = friend
callerid = User 1001 <1001>
secret = 1001
host = dynamic
canreinvite = no
dtmfmode = rfc2833
mailbox = 1001
disallow = all
allow = ulaw
transport = udp

[1002]
type = friend
callerid = User 1002 <1002>
secret = 1002
host = dynamic
canreinvite = no
dtmfmode = rfc2833
mailbox = 1002
disallow = all
allow = ulaw
transport = udp

edit /etc/asterisk/extensions.conf

[general]
static=yes
writeprotect=no

[default]
exten => 1001,1,Answer()
exten => 1001,n,Dial(SIP/1001,20,tr)
exten => 1001,n,Hangup

exten => 1002,1,Answer()
exten => 1002,n,Dial(SIP/1002,20,tr)
exten => 1002,n,Hangup.

Soft Phones e.g. Ekiga or Jitsi