Cell Phone Spam, Unwanted Ads

I have a question that I'm sure A lot if people want to know. Can someone Please Help. Who gives people and companies the right to SMS and phone us on our cell numbers, since as far as I know, No permission was given to do so, unless as usual it's the "fine print" These are So Annoying as I may be expecting Important calls or sms's. Does it mean that cell phone companies are selling our numbers to all these companies.

I take out a contract to stay in touch with people, Not to Receive All This Constant *******.How do We Stop receiving all these Unwanted Sms's Regards,