#########WITHOUT CALLBACKEXTENSION
sip.conf
[teste]
type=peer
description=teste
dtmfmode=rfc2833
qualify=yes
directmedia=no
nat=force_rport,comedia
port=5090
disallow=all
allow=alaw,ulaw,ilbc,gsm,g722
host=200.200.200.200
defaultuser=teste
fromuser=teste
secret=XXXXXXXXXXX
insecure=port,invite
srvlookup=no

########
Cli> sip show peers
Name/username             Host                   Dyn  Forcerport Comedia    ACL Port     Status      Description                      
teste/teste          200.200.200.200             Yes        Yes         A  5090     OK (9 ms)   teste

#######WITH CALLBACKEXTENSION
[teste]
type=peer
description=teste
dtmfmode=rfc2833
qualify=yes
directmedia=no
nat=force_rport,comedia
port=5090
disallow=all
allow=alaw,ulaw,ilbc,gsm,g722
host=200.200.200.200
defaultuser=teste
fromuser=teste
secret=XXXXXXXXXXX
insecure=port,invite
callbackextension=teste
srvlookup=no

Cli> sip show peers
Name/username             Host                   Dyn  Forcerport Comedia    ACL Port     Status      Description                      
teste/teste          200.200.200.200             Yes        Yes         A  5060     OK (8 ms)   teste

CLI> sip show registry 
Host                                dnsmgr Username       Refresh State                Reg.Time                 
200.200.200.200:5060                 Y      teste         105     Registered           Wed, 29 Mar 2017 13:37:52