Keep in mind if you have a four digit and a five digit like the example in Ronsters message you will get complaints by the users. heres why,
if you dial extension 10000, the call will go thru to the extension no problem, but if you dial 1000, you will have to either end it with a pound or wait a few seconds for the definity to realize you have completed dialing. People will complain that there calls are not getting thru.
I would go with the second digit scenario. you more than likely have to rebuild the station dialplan to make this work right.