Hi mennotrav.
Thank you for using the forum.
You need to turn on fax audio line monitoring from tools but it requires admin access.
Here is the solution that you might take a look at. It may offer more detail.
How To Enable Fax Audio Line Monitor
If that does not resolve your issue you may consider contacting Support at the following link: Fast Track to the Expert or by phone 1-800-821-2797 U.S. & Canada Mon - Fri 8AM-8PM ET.