Have tried to do what you did.
In Axon I added an external line called skype.
I have to supply a server ip, uplink showed my 10.0.0.200:5071 as sip private ip so I used this.
Axon requires to supply username and password. I used uplink in both cases but have tried skype and other names.
Using dialing plans i for example used 88 using remove 2 digits and than dial out using skype.
When I dial 88 uplink shows an incoming sip call.
When I dial 88echo123 (test service) Axon shows me an error message
19:46:16 Call 124 -> sip:88echo123@10.0.0.200
19:46:16 Authorization failed for server
19:46:16 Error message was: Proxy Authentication Required
19:46:16 Check username or password for the proxy server.
19:46:16 Perhaps calls to this number are not allowed by this server.
19:46:16 Call 124 -> sip:88echo123@10.0.0.200 disconnected
For test call I am using express talk