Block extensions from calling each others

Hi all,
i installed freepbx and i am willing to provide services for some clients , and i don’t want my clients who have different extensions to call each others, let’s say like 101 , 102 , 2010 , 2011 , the extension 101 can call 102 but not 2010 and 2010 etc … , so how can i do it , i used extensions_custom.conf and put :

[from-internal-custom]
exten => 102,1,GotoIf($["${CALLERID(num)}" = “104”]?from-internal,102,1)
exten => h,1,Hangup()

it seems that the context from-internal can not be called from within the extensions_custom.conf either my sentence is wrong.
Please any advice or help is appreciated.
Regards;

See How to give a particular extension different or restricted trunk access for outgoing calls and scroll down on that page to the section heading “What about restricting calls between extensions on the same FreePBX server?”