In the website/funnel builder we have a URL/Button action of "send SMS".

You enter the number and when clicked the user's phone goes to their text messages and opens a new blank message.

If you do this with a link you can prepopulate the message for them so all they have to do is click send.

By adding a text field in the dialog we could carry the message body when clicked.

This would be perfect for QR codes, too - they scan QR code and it opens a prepopulated text message and they just click send.