extensions.conf:

[from-internal]
exten=>6001,1,Dial(SIP/demo-alice,20)
exten=>6002,1,Dial(SIP/demo-bob,20)


sip.conf:

[general]
context=internal
allowguest=no
allowoverlap=no
bindport=5060
bindaddr=0.0.0.0
srvlookup=no
disallow=all
allow=ulaw
alwaysauthreject=yes
directmedia=yes
nat=force_rport,comedia
session-timers=refuse
localnet=192.168.1.0/255.255.255.0

[demo_alice]
type=friend
host=dynamic
secret=123
context=internal

[demo_bob]
type=friend
host=dynamic
secret=456
context=internal