I have written the sem-official guide to setting up the SPA 3102 with Axon. (It is a pinned topic in this forum). NCH then took my guide and used it as the basis of their new guide for setting up the adapter (found on the NCH website under hardware support). Either method should work, and they differ only in a couple of areas.
So I suggest you refer to one of those guides first and then see if you can get it working.
For the dial plan you'll be able to set it up so when the number starts with 0, it can be forwarded to the physical exchange (your PSTN line), otherwise use another line or dial the number as an extension.
You'll also be able to configure it so any calls that come into the SPA 3102 FXO adapter will be sent through Axon and will ring the a chosen phones throughought the campus.
If you need more information just let me know.