NationBuilder pages frequently prompt folks to leave their cell phone numbers for mobile outreach and followup. For many campaigns and initiatives, text-based outreach can be very powerful. You can use the Messages tab to select a broadcaster and Texting to do simple mass texting campaigns. If you haven't already set up a phone number, you'll be prompted to do so before you begin a text campaign. You'll also need a texting plan (Settings > Billing & add-ons). Folks you contact will be able to text back and also to call the number (voicemails will come right back to your NationBuilder Dashboard and Messages sections).
Here's a video guide on setting up and using text messages - nationbuilder.com/how_to_text_messages