asterisk users - Dec 2004 - dialplan "not ${VARIABLE}

I try to use my dialplan more general usable, ...
I defined the varable: 
COUNTRYCODE=886  (Taiwan)
LOCALAREA=2  (Taipei)


To avoid any changes in the dialplan I would like to NOT allow to dial 
the areacode I am in:

Is something like    _9(N-${LOCALAREA})     available?
(above should give me 9[3456789]  and in LOCALAREA=4: 9[2356789])

Thanks for your hint!

bye

Ronald