In Axon's web config,
Dialing Plans -> Default -> Add Dial Rule
If number starts with: 9
-> Remove Digits: 1
-> Prepend:
-> Dial on line: Your <VoIP Line>
-> Save Changes
If none of the above applies, dial on line: [Extension]
-> Save Changes
This is an example of a dial plan that will send calls out to your VoIP line if they start with the digit 9, and send all other calls to their respective extension destination. You can also make a dial-plan that works in the opposite way (all calls go out on external VoIP line except if they begin with a particular digit, in which case they are routed to the extension instead).