In this case, I think they have an established status quo, with different providers being the clear winner in certain markets, so they don't compete directly. At least, not very much.
Just as an example, many tried to shorten the "month" to 28 days and they got a combined €228M fine in early 2020: https://www.repubblica.it/economia/2020/01/31/news/bollette_...
€228M for essentially a 7% hike in pricing.