I have, despite the partly bad reviews decided to Fritz DECT Repeater to buy in order to test him at least. We have in the basement since more than 1 month, a Fritzbox 7490 with FritzOS 6:24 since today (Previously Fritz box 7390 and before that two others, I'm always a fan of Fritz hardware). On the Fritz box are Dect 3 FritzFon are connected: 1 on the ground floor, 1 on the first floor, 1 in the attic. So far, the contact especially in the study in the attic was extremely poor until completely gone. The problems are described here in various commentaries. Whistling and otherwise poor sound - on all phones - have been repeatedly criticized by callers. A strange DECT repeater, which was also set up cryptic, brought no improvement. The Fritz repeater was actually set up against it in 1 minute: Repeater into the outlet, placed next to the Fritz box, pressed DECT button, wait 1 minute, finished! Then I plugged it on the first floor and since then run all three phones completely trouble-free and 100% reception. So have nothing to complain about, 5 stars.
Addendum: I've actually tested only after sending the criticism, whether on a call only 2 phones ringing of the 3, as described in a review. But even that is not the case with us. It will ring every 3 each in succession, which is perfectly ok, so still 5 stars!