I am testing AXON, and while I like it because it's slim and fast, it has many incompatibilities.
Some example:
with VOXOX (sip server/proxy: sip01-west.voxox.com) axon registers but it's unable to place a call and gets bad auth (try it with a toll free number 1800xxxyyyy)
doing the same with any soft phone works without a problem.
with provider italiadsl which uses hbcall.com on sip server 188.138.16.172 it's impossible to register.
On Eutelia (www.euteliavoip.com) everything works but if the incoming call has NO callerid AXON refuses the call.