SMS notifications on pageviews with Segment and JavaScript, using Twilio Serverless, allow developers to send SMS messages when a user visits their website. To achieve this, developers need to create a Segment source for their website, set up a Twilio function using the Serverless Toolkit, and configure Segment as a destination for the Twilio function. The process involves adding a JavaScript file to the website's `<head>` element, creating a Twilio function that sends an SMS when the website is viewed, and setting up a webhook in Segment to connect the two services.