Asterisk: Failed to authenticate

What it means

A device (or someone) tried to register or call with a username that exists but a wrong password.

A phone or softphone with an old password.
Password guessing from the internet: many attempts from one IP for different extensions.
Auth username differs from the extension (auth section username).

Compare the password in the phone with the type=auth section; re-provision the phone.
Many attempts from foreign IPs: block them, enable fail2ban with the asterisk jail, use long random passwords.
Never expose 5060 to the whole internet without a firewall allow-list.