If you mean each Telstra line you have is set-up as a different external line in Axon, then you'll need to set-up a dial-plan which allows you to select the outbound line to use when dialling a number.
For example, you could have a dial plan where numbers starting with 1 go out on line one, 2 to line two and so on. Then you could select which line to use when you dial outbound calls so that the number corresponds with the extension you are using. Note that all of Linksys/Sipura's products (to my knowledge) support dial plans that let you prepend digits in front of numbers. You could have each Linksys/Sipura phone or adaptor you have automatically dial the corresponding prepend digit prior to making each outbound call so that the correct line is used based on the extension that is calling.
On a side note, spoofing is a technique used to mask your true Caller ID with another name/number of your choice. On many VoIP systems this is very easy to do, the client (you) digitally sends the outbound caller ID information to the VoIP service which then sends that on to the analogue phone networks and ultimately to the party you call. On traditional analogue systems there is no way to spoof your Caller ID or send a new Caller ID to the telco. Instead, the telco manages everything for you and has complete control over what Caller ID is used. The only known ways of spoofing analogue networks are:
- Dialling into another phone system and gaining access to their dial-tone and making a call from their line
- Calling into a VoIP system which can transfer your call to another number using a Caller ID of your choice
Also on a side note, spoofing has many legal restrictions and in some regions of the world it is now illegal (even if you also own the number you are spoofing)