If you are using a Cardserver for recording, from AMTCards version 1.6.3.11 the Extension-side solutions default the username to "EXTxxxx", where "xxxx" is the extension number. This helps sites distinguish between recordings on extensions where the username is not set, and also helps managing Supervisor access for sites that have high staff turnover.
If no user name is set it will thus show as EXT followed by the physical extension number the recording was made on.
You can configure the usernames under 'Configure Virtual Extension',
refer to topic:
How do I configure virtual extensions in AMETHYST?NOTE: Extensions already added to the list cannot be removed, you can clear them by removing the user name next to.