I didn't purchase a texting plan, yet...

I have to wait a little over a year until my original plan. I didn't add texting yet because I wanted to have a little bit of an idea what it would cost me overall. However, I have apps that work for my iPhone, but not with my mom's HTC cell phone.

Is there any kind of app that I could use that is preferably free to text her with that she doesn't need the app on her phone? I keep reading about apps like this that assign numbers and I don't want an additional number. I just want to use my original number for texting as well.

Thank you!! :)