This feature depends on your PBX.
In a typical situation you might push "0" in order to access an outgoing line.

NOW you'll simply configure your PBX to use a different prefix (Example "9") in order to automatically select an outgoing Skype line.

Another interesting way to go is by using LCR (Least Call Routing), so that your PBX automatically recognizes and selects the better option depending on the telephone number dialed.

It can all be decided based on the functionality and configurations of your PBX.