Why do I receive different casino advertisements via SMS every month?

Good day,

A brief history: I once gave my information to a colleague so he could create an account on a casino site so he could get a bonus by depositing €100 and playing with €1,000 or something like that. However, we've already had the account deleted from the site, turned off all notifications, and spoken to support about it.

The problem now is that I keep getting text messages from casino sites, even though I've never registered, played, or used any of them. The text messages contain an offer and a link to the site. There's also an option to STOP to stop receiving messages, but that doesn't work.
Whenever I contact support, they just tell me that I don't have a registered account and they won't send the text messages. After that, the text messages from that site stop, and I get text messages from another site. And the whole thing repeats itself.

I can't take it anymore. I don't want to contact the new site's support every time, only for it to continue like this. How do these constantly appearing new casino sites get my number? Does anyone know how I can stop this for good?

Thanks in advance!