Classtools SMS Generator is free to use and does not require students to log-in. To use the SMS Generator just click the left speech bubble icon and enter a message. Then to create a reply just click the right speech bubble icon and enter a new message. You can make the exchange as long as you like. To share the conversation click the sprocket icon and grab the embed code, direct link, or QR code for the exchange.
Applications for Education
You could have students use the Classtools SMS Generator to create simple conversations between historical characters as way to get them to think about those peoples' lives and the conversations that they might have had.