To link your VoIP provider with Axon, open the Axon web configuration panel and add a new 'External Line'. Fill in all the details (username/password/server) given to you by your VoIP provider. Once you've saved the configuration, Axon should be ready to make/receive calls.
The next thing to do is set-up some extensions. This is pretty straight forward, add a new extension, give it username/number and a password and then you can set-up an IP phone or Soft Phone (e.g. Express Talk) with those details and they should be ready to phone out & accept calls through Axon.
The next thing to do is to add all the extensions to a ring 'Group' so that incoming calls will ring all the extensions instead of just one (you can pick and choose which extensions to ring). Once you've done that, simply go back to the external line settings and set the line to send incoming calls to the ring group you just created.
[incoming calls should now ring as expected]
The last step is to configure a dial plan. Again this is pretty straight-forward, you can basically add rules so that calls starting with a certain digit go out on a certain line. For example, you could set a rule where calls starting with 1 use the internal Axon [Extension] line to call other extensions, and make all other calls go out onto your VoIP line.
[Outgoing and Internal calls should now be working]
That's it.
The only thing is you mentioned PSTN. Generally speaking, PSTN is a term to describe analogue telephony systems and not VoIP. Do you wish to connect normal analogue lines to Axon as well as VoIP lines?