It's a pretty common problem. Spammers, don't need you to give them your number, they just randomly generate them. I had a big problem with SMS spam with my old phone. But somehow when I switched to my iPhone it went away. I still have the same number even.
Anyway, if you list your cell phone number on the Do Not Call Registry, it may help. Doing this cut my spam quite a bit.
https://donotcall.gov/