asterisk users - Feb 2006 - Dumb question... block 00

hi all a dumb question..

how i do to block the 00 for certain sips extensions? 

for example i have the extensions 400 to 500
i need to extension higher than 429 can't digit 00 

in my extensions.conf i have

exten => 420,1,Dial(SIP/420,20)
exten => 420,2,Hangup
exten => 421,1,Dial(SIP/421,20)
exten => 421,2,Hangup

exten => 430,1,Dial(SIP/430,20)
exten => 430,2,Hangup
exten => 431,1,Dial(SIP/431,20)
exten => 431,2,Hangup


is that possible?

thanks a lot.

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