Setup a form to capture the message detail, and sender info.
Connect that form to a webhook, on a service like Make or Zapier
Send the SMS from that automation
Note, in many countries / networks, SMSs are not free to send, so you may need to setup an account with an SMS provider for those sends. I use ClickSend, but there are many.
Webflow Logic may be able to provide this at some point, I’d check there for SMS capability as well, as it develops.