Vodafone, how do they know if I'm using HotSpot?

Question

Google Chrome

Good evening. Recently I had to buy a new package, they offer 3G and 3GB for a day, but here a problem arises, since it is forbidden to use the phone as an access point or in a modem.
Actually, the question arises, how did you know that I use HotSpot on my phone?

Answer 1

Via TTL usually.