According to a good friend of mine at AVM, all the latest firmwares add T.38 support to all of the Fritz!Box models. Unfortunately I was only able to test the following models so far and confirm that T.38 actually works:

• Fritz!Box Fon WLAN 7270 (German edition / Annex B)
• Fritz!Box Fon WLAN 7170 (German edition / Annex B)
• Fritz!Box Fon WLAN 7170 (Int’l edition / Annex B)

I would like to test some more models, like the 5170 and 5188. Well let’s see if I find enough time.