Shortcoming as AVM products unfortunately mainly: the range of the wireless performance in both the WiFi and DECT. Already overcome a floor with reinforced concrete slabs, is a challenge. If the box is not right in the EC you get problems in the DG or KG to the corner rooms reasonable reception. I've helped me with a WLAN Fritz repeater and TP-Link Powerline adapters. In terms of DECT I had only a HAMA DECT repeater in use, but repeatedly caused problems. Moreover, with this encrypted connection was possible, nor the transfer of caller images on the display of the Fritz-Fons. Using modified placement of the Fritz box I could anywhere in the house at least recorded, even without repeaters.
There are so few alternatives to Frtizbox if you want combine Internet and telephony in a single device, to my knowledge, except the Fritz boxes only Speedports Telekom. Therefore, I stay with the Firtzboxen.