check the screenshots below--those are the settings for the phone in my core. I don't know however if you can actually create a working extension there, or if it all has to be done by script. On the other hand maybe its worth a shot
just to make sure we are talking about the same thing: an are code is in front of a phonenumber, or local number. So if you want to make a local call, like calling your neighbor, then you would only need to dial the local number without countrycode or areacode, right?
for example a number in germany with canoncial format would look like this : 0049 1111 222222 (countrycode areacode localphonenumber)
As far as I know there is no local number with only three digits. The only three digit numbers are emergency call numbers like 911,110, 115, etc. That is something the dialplan can take care of, if Asterisk doesn't.
Just for reference check http://en.wikipedia.org/wiki/Telephone_numbers_in_Europe
Anyways, thats the way I have set it up and so far its working quite nicely