There are many settings within the fritz box that can cause echos. I have a linksys adaptor that is a similar device and there is an actual menu that mentions echo with settings from 1 to 10 that can be changed.
There are many forums on the net dedicated to the fritz box that may help. very unlikely to be a line fault.
I suspect every time an engineer attends and can't find a fault you will be charged an abortive visit.
(If I have helped you in any way please click the thumbs up. Thank You)
If I have solved your Issue please click the "Mark as accepted solution" button.