Markdevoll,
First of all you need to make sure that you carrier allows you to control it. After that it is just a matter of putting a 10-digit number in the extensions Calling Party Number field or the CO Trunk Group Calling Party Number Field. Also don't forget that if you are sending out a number that isn't known to the ANI/ALI database then 911 may not know where to go so make sure you have the main number in the Emergency Calling Party Number so they know where to find the person.
Thanks,
TE