We guarantee the IP Phones at https://www.nch.com.au/hardware/ipphones.html and FXO devices at https://www.nch.com.au/hardware/fxo.html will work with Axon. There are of course other models and unfortunately we cannot test them all. So if in doubt we recommend you buy from the above link, or failing that you should buy a similar product from the same companies mentioned on those pages.
If you do find another device which either works or has problems let us know by posting it on the Axon Forum to help others.