The Price It by Phone service allows users to comparison shop for books using their touch-tone phone by calling a specific number and entering the 10-digit ISBN code of the book they want to check prices on. The service sends a REST request to Amazon's Product Advertising API, passing in the ISBN number, and receives a response with the lowest new and used prices for the book. The user can then enter their phone number on the web form to view a list of books they've already price-checked, with links to their Amazon detail pages. The service was built using Twilio's Weather By Phone App Engine sample as a starting point, and Audrey Speaks, the developer, completed the project in just one weekend, citing that the Twilio API was easy to use and that dealing with Amazon's XML response was challenging but manageable.