Ensure agents PIN and EXTENSION are both correct
(Using the desk extension for in office agents, and their assigned extension for WFH agents)
Try another extension – by either getting an extension from Tom Ballentine's document if they are working from home, or an empty desk to test if it then connects with no issues.
If it connects, it was an extension problem, if it doesn’t you will have to default the settings as normal and try again.
To default the settings in Avaya, please action the following:
https://resqcs.atlassian.net/servicedesk/customer/portal/1/topic/6e9cf837-98c2-40b5-bea1-bf7516779aa9/article/526581761
If you are still having issues, please raise a ticket.