If you've activated the SMS & Inbound Calling addon, you can send text messages manually and automatically in LeadSimple. Learn more about how to do this here.
Once you've texted a lead, however, you want to know when they text you back. You'll get an email notification from LeadSimple in your inbox with the content of their text. You can reply to them right there in your email inbox and they'll get your reply on their phone as a text.
Alternatively, you can go into LeadSimple and text them back from the lead's page. LeadSimple will also track all the texts back and forth within the lead's page.