Thanks for posting your query however seem like you have posted on it on wrong forum. This query seems relavent to Avaya Breeze product whereas this forum is designed to answer queries for CLIENTSDK only.
Kindly post your query Breeze forum on https://www.devconnectprogram.com/forums/forums/show/18.page

Thanks,
Avaya DevConnect support team

Hi Børge,
Thanks for posting query here

Here is we think might help you-
User object creation will not fail if incorrect extension / password is passed.
When User.Start() is called, Client SDK attempts to start all configured services. If incorrect extension / password is provided, then Client SDK provides OnUserRegistrationFailed, OnUserAllRegistrationsFailed callbacks on UserRegistrationListener.
This can be used for detection of login failure.

regards,
Avaya DevConnect Team (CSDK)