[general]
 context=default
 port=5060

 maxexpirey=3600
 defaultexpirey=120

 language=en
 nat=yes
 canreinvite=no

 disallow=all
 allow=ulaw
 allow=alaw

 dtmfmode=rfc2833

 [20] ;X-lite client 20
 type=friend
 host=dynamic
  context=internal


 [21] ;X-lite client 21
 type=friend
 host=dynamic
 context=internal

[root@localhost asterisk]# cat extensions.conf

 [general]
 static=yes
 writeprotect=no


 [internal]

 exten => _XX,1,Dial(SIP/${EXTEN})