A SIP extension is configured in the SIP channel driver configuration file, called sip.conf. (This file resides in the Asterisk configuration directory, which is typically /etc/asterisk.)
Open sip.conf with your favorite text editor, scroll to the bottom of the file, and add a section for your extension. You’ll need to choose your own unique password for each account, and change the permit line to match the settings for your local network.

For example, to configure an extension 1500, following lines needs to be added to sip.conf:
[1500] deny=0.0.0.0/0.0.0.0
type=friend
secret=****
qualify=yes
port=5060
pickupgroup=
permit=0.0.0.0/0.0.0.0
nat=yes
mailbox=1500@device
host=dynamic
dtmfmode=rfc2833
dial=SIP/1500
context=from-internal
canreinvite=no
callgroup=
callerid=device
accountcode=
call-limit=50

Make sure to restart Asterisk in order for these changes to take effect.