There's a lot of things you can say about that, but it's arguably not really a "GPL violation" as commonly understood, and a single incident in almost 40 years of history doesn't make a company "notorious".
The freedom to take a device and modify it to run your own changes is what the GPL is all about. Let's not forget the modern open source movement was the result of someone not being able to load a custom font into a printer. Do you think Stallman would've been satisfied if Xerox had given him the source code he asked for, but then indicated they'd sue him if he tried to load any modified software onto the printer?
AVM's interpretation of the license ("you can modify the GPL software but you're not allowed to flash it onto the device") was shot down by the court, and rightly so.
I think a company actively going after another to prevent exercising the software freedoms granted by the GPL makes them deserving of being called "notorious". It's a shitty practice in general, but it's especially shitty when you do it for GPL firmware.
I can't find anything on that.
(and it's not that I disbelieve you, but details do matter, so very high-level descriptions isn't something I can really do anything with).
I just read up on the details, as best as I could with machine translation and my terrible German anyway.
Basically, a couple of thousand modems made for ISPs that got sold off after replacement ended up getting bought by Woog, which then flashed the latest official firmware for the hardware (which wasn't distinct from the carrier hardware in any way) into them. AVM didn't want them to sell carrier modems with unlocked firmware.
When I read about this stuff first, it was all labeled "custom firmware", but it looks like the firmware wasn't so custom after all.
Either way, AVM sued a company selling second hand hardware and now a tens thousands of perfectly functional modems seemingly ended up in a landfill somewhere.
I kind of understand AVM's viewpoints on that one because they probably had a special deal with the ISP to sell these modems at a cheap price and now they're basically competing against themselves, but scrapping thousands of perfectly functional devices is of course just silly. My fritzbox had a "eco mode" or some such and I'm pretty sure that the scrapping completely obliterated the savings of eco mode of all fritzboxes combined.
I'm not hugely impressed by either case, but I also don't really think it backs up the "AVM is notorious for GPL violations" and "they intentionally violated the patents" (with an implied "they just ignore any IP law they don't like").
See https://github.com/Freetz-NG/freetz-ng/discussions/134 (and possibly translate it).
Unfortunately, the DECT spec as implemented by many manufacturers isn't as broad as people seem to think it is. Most interoperability was done through DECT GAP last time I checked, and GAP lacks common features like "rich caller ID" and "address books".
I've seen other brands do address book support just fine, I think AVM tries to be compatible enough but just can't get the full feature set for every brand and model.
I've even seen Gigaset phones failing to support address books on more modern Gigaset basestations, DECT hardware is kind of a mess in that way.