Callbackextension just adds simplicity and convenience.
All the settings are now in one place - in the peer definition.
No need to specify host twice. No need to specify username twice.
No need to specify your password (secret) again.
No need to expose your password in a clear text - now you can use md5secret for both calls and registration.
This works fine with UDP transport.
For TCP and TLS transport this either does not work at all or nobody wants to open a secret 
My question without answer: TCP/TLS register with md5secret