How to use custom contex module to allow only specific dialplan rule for users

hi ,
my system info is ,

Kernel
Linux(i386)-2.6.18-238.12.1.el5

Elastix
elastix-2.2.0-14
elastix-firstboot-2.2.0-5
elastix-system-2.2.0-14
elastix-email_admin-2.2.0-9
elastix-vtigercrm-5.1.0-8
elastix-extras-2.0.4-4
elastix-asterisk-sounds-1.2.3-1
elastix-simmrate-2.2.0-10
elastix-my_extension-2.2.0-5
elastix-agenda-2.2.0-5
elastix-a2billing-1.8.1-16
elastix-addons-2.2.0-4
elastix-im-2.0.4-2
elastix-pbx-2.2.0-14
elastix-security-2.2.0-7
elastix-reports-2.2.0-6
elastix-fax-2.2.0-4

RounCubeMail
RoundCubeMail-0.3.1-10

Mail
postfix-2.3.3-2.3.el5_6
cyrus-imapd-2.3.7-12.el5_7.1

IM
openfire-3.5.1-2

FreePBX
freePBX-2.8.1-7

Asterisk
asterisk-1.8.7.0-0
asterisk-perl-0.10-2
asterisk-addons-1.8.7.0-0

FAX
hylafax-4.3.10-2rhel5
iaxmodem-1.2.0-1.1

DRIVERS
dahdi-2.4.1.2-5
rhino-0.99.4-2.rc1
wanpipe-util-3.5.23-1

my question is , i want an extention to allow it to dial only a specific extentions and i want to deny him to every else.

as an example ,

i have extention 200

i want to deny channel spy 555 and outbound calls and only allow him to call either 300 or 400 or 500 .

how to do that is custom context module ??

hint i installed it and create a custom but need help what choices i need to allow and permit to satisfy my rule .

with my best regards