Something

sip.conf
[general]
guestallow-yes

[bill]
disallow=all
allow=alaw
allow=ulaw
type=friend
secret=welcome
qualify=yes           ; Qualify peer is no more than 2000 ms away
nat=yes                ; This phone is not natted
host=dynamic          ; This device registers with us
canreinvite=no        ; Asterisk by default tries to redirect
context=internal



extensions.conf
[internal]
exten => 300,1,Dial(SIP/bill)