This has become a very real problem on the internet...
our information is valuable and these sites have farmed it before we realize it. If I get into any site asking for all my personal info before I can get a cost, I drop it like a hot potato. If it is something I really want, I will call to see if I can find out pricing (and the outrageous shipping charges) and order over the phone. If they won't tell me, I tell them why I will not be buying anything from their website and hang up.
I've also stopped giving out my mobile to anyone unless I want them to have it. If I don't recognize the number, I don't answer. Someone I know will leave a message and I can add the number to my contacts.
If the person/company doesn't need my number but is farming it to sell, that is obvious and I just make up something. If it's a vendor I'm actually doing business with, we have an old landline on voicemail to collect those calls once/week. It's never answered otherwise.
Far too many robo-calls on mobiles these days and it is supposed to be illegal.