As a part of my project, I sometimes need to register or buy some products that requires phone verification or text messages. Even though it's a guarantee from them that they will never "use your phone number for commercial purpose", but who knows. I've been receiving a lot of phone call, especially from those domain/hosting providers like 1and1, dreamhost, digitalocean etc... and a lot from other advertising firms that I don't really care.
Knowing that it would happen sooner or later, I've prepared myself by using Google Voice (sound like i'm advertizing for them ^^ but i've tried Skype before and they didn't meet my expectation). So instead of giving out your real phone number, you could get yourself a google voice number.
How it register? Well just follow all the step on Google Voice service, and link your phone to a Google Voice Number which you can choose. (good thing about this is you can actually search for your actual sequences that you like - So like my last name is NGUYEN, which corresponds to 64896, I searched for that sequences and got myself a nice 555-666-4896, or 555-66-NGUYEN.)
How does it help you to avoid unwanted call? So whenever you are registering for a product that requires a phone or text verification, just use your google voice, they will actually reach your voice mail or text you by the number that you have registered with Google. Sometimes they call and you don't want to listen, then just listen to the voice mail only :). You can actually translate those voice messages into text, which is really cool, or just simply give them a call back / text by using that google voice number. :D