You have to setup a dest code for the number that is being dialed with absorb length none. This allows you send the call out without dialing a prefix. The only thing that could get in the way is if you had an extension that matched the first digits of the number. For example, if you are dialing a 1800#, you would first have to change extension 1800 to a different number before you could build the dest code.
hope this helps
rob