Giving your phone number is just as bad. I was buying stuff at World Market and they had big signs touting 20% off some things... but when you got the counter they told you didn't get that unless you coughed up your real working mobile number so you could receive some BS code.
There are actually phone numbers that post all text messages they receive online automatically. Personally this has never worked for me though because all the services I tried to sign up for someone else had already used that number
See ya, jerks.