My VoIP provider is exactly the same.
I tried put in a random user/password but provider's server returns error "405 Method not Allowed" on registration attempt.
I've set up Axon this way:
1. Add new external line of type "Other device will contact us (eg. FXO, UpLink, etc...)" whith desired Line Name, any "ID or User Name" and empty password
2. At the computer running Axon modify registry entry
HKEY_LOCAL_MACHINE\SOFTWARE\NCH Swift Sound\Axon\Lines\External\EXTERNALLINENAME\Server, enter IP addres/FQDN of VoIP provider.
Axon works well for outgoing calls. I can't tried ingoing - my line is outgoing only whithout PSTN number.
One small problem. Even Axon restarts or eny external line reconfigured I need repeat theese steps!
It will be so fine if Axon team imlements external lines without autentification.
