If you log into your Fritz!Box at http://fritz.box/html/capture.html
and select start on '1.internet connection'
Leave this running for 30secs. or so, and open the resulting file in wireshark. Filter the results by ICMP.
(also select Resolve IP addresses' in preferences)
You should see the TBB pingbox1 pings & response, 1 every second. Anything else?