Or 4. This forum community is not very active, and a significant majority of the (small) userbase are novices and probably don't know the answer.
Normally I would help out, but I haven't used Axon in over a year now and I have never experienced this error before.
1. Check to make sure the dialplan appears under 'Dialing Plans' on the Axon web config (I assume you've got this far)
2. Make sure you select a default dialplan (If I remember correctly, there used to be a drop down menu at the bottom of the dialing plans page
3. Make sure the dial plans are not excluding the extension you are dialing from
4. If Axon allows you to chose a dialplan for each extension now, make sure the correct dialplan is chosen
Can't really suggest much else.