The '9' prefix to reach an outside line likely corresponds to a specific outbound route that handles phone numbers matching a certain regex (generally length).

The '9' prefix to reach an outside line likely corresponds to a specific outbound route that handles phone numbers matching a certain regex (generally length). Depending on how outbound routes were configured on the PBX a prefix might not be required for the outbound route set up with regex to match 911. I think this is often done as a safety feature, so somebody not knowing or forgetting the exit number can still dial 911 in an emergency and reach emergency services. I've seen PBXs set up this way in past.