sip.conf

[general]
port=5060
bindaddr=0.0.0.0
context=default
language=de
disallow=all
allow=alaw,ulaw  ;alaw,ulaw,g726
dtmfmode=info
;relaxdtmf=yes
nat=no
canreinvite=no

[868]
callerid=868
type=friend
host=dynamic
secret=password
context=interview
permit=0.0.0.0/0.0.0.0

[441]
callerid=441
type=friend
host=dynamic
secret=password
context=office
permit=0.0.0.0/0.0.0.0

[gsmgate01]
type=friend
secret=password
call-limit=8
callerid=GSM Gateway
nat=no
host=dynamic
port=5060
insecure=port,invite
canreinvite=no
context=interview


extensions.conf

[interview]
exten => _9980ZXX!, 1, answer()
exten => _9980ZXX!, n, playtones(waitforring)
exten => _9980ZXX!, n, dial(SIP/${EXTEN:3}@gsmgate01:5060&Local/s@no-op,60)

[office]
exten => _786X, 1, chanspy(SIP/${EXTEN:1},qv(1))

[no-op]
exten => s, 1, Hangup()