In salesforce flow develop an apex module that can receive from the flow variables: sent from Name, sent from phone number, Sent to phone number, body of text. See Picture 1.
Receive all return codes from twilio back to salesforce including but not limited to Status, Error Code, Error Message, Direction, ApiVersion, Uri, SubresourceUri, etc.
Modify the code at http://www.clintslee.com/2011/02/08/salesforce-twilio-receiving-incoming-texts/
Use the newest REST API: Messages https://www.twilio.com/docs/api/rest/message on the twilio site.
The apex code should limit the number of message sent to no more than 1 every 2 seconds.
Body of text should be a Text Area (Long) field.