That sounds like a robot dialler. The main symtom of these is the disconnect after 2 seconds when you answer (due to the fact it cannot forward the connection to a sales agent).
I am sure it is against casino T & C to use a Robot, so they seem to be breaching their own T & C
It does not mean they necessarily have your number, these robots cycle through all possible number combinations till they get lucky, one way to tell is that if connected to an agent they will not be able to address you by name. If they call you by name, then they have a list of names & numbers.
If this is a marketing call from an online SPORTSBOOK, from a US phone number, you should be able to make a complaint since sports betting by phone is one thing that IS illegal. At best, they will have the phone number confiscated as they are marketing an illegal activity.
This trick is often tried here in the UK, and usually the number comes from offshore, so our own authorities have trouble chasing them. If the caller is using a number regulated by our own authorities, or those in the EU, we can make a complaint based not just on marketing an unlawful activity, but breach of privacy laws if our number has been registered as "do not call" for marketing purposes.