Spam Text Messages

So my mom has a cell phone that she can kind of use. She make calls and answers them. that's about it. Forget checking text messages, or voicemail.

Now me, I only have a cell phone - no lan line at all. My cell phone is on 24/7/365. I never have it off. My mom on the other hand *only* turns it on when she wants to call someone and she's not home. Which really defeats the purpose of the cell phone - because we can never get in touch with her - since the phone is off all the time. What is the point? I think she feels like she is saving the battery, but just charge the damn thing and be done with it.

Anyway, that's not really the point... last night she shows me her phone saying I am getting text messages from people I don't know. How can I stop it, and am I getting charged for it? I said no you can't 'block' numbers and yes you get charged. So I checked and sure enough besides one text message from me she had about 10 spam messages! How does this happen? I have never once gotten a spam text message. Plenty of spam email, but not texts.

I suggested she put herself on the do not call list. She said she is. So I am stumped - how do people get her cell number to spam her?