Cellphone numbers can very easily be abused to steal money (premium SMS), to steal my identity, to spam me in the middle of the night, to pull me out of the "zone" by calling me/messaging me during work hours, to track my location while roaming and probably a lot more stuff that's not currently apparent to me.
Also, my phone number is known to some identity providers I trust. If they sent me an SMS asking me to click a link, I might be inclined to follow that link. This is quite right despite the sender being very easily spoofable because nobody but these identity providers know my phone number.
It is ridiculous to think that your number is private and even moreso to think that someone can steal money with just your cellphone number.
Of course you could be phished or tricked by SMS but to expect your number to be private is to expect everyone ever who you give the number to go to extreme to keep it private as well.
If you ever gave your number to someone who downloaded an app which has permission to contacts your number is no longer private (Facebook has taken big advantage of this, I'm sure there are less than reliable apps that took bigger advantage).
It's actually fairly easy in some cases. If the user is with Virgin mobile, their 6 digit password can be bruteforced in a couple of minutes, and then every text and call they've ever made is public. I've made a fuss to them about this quite a few times, but they've never done anything to fix it.
Also, my phone number is known to some identity providers I trust. If they sent me an SMS asking me to click a link, I might be inclined to follow that link. This is quite right despite the sender being very easily spoofable because nobody but these identity providers know my phone number.
No. I'd rather be very careful with that number.