Twilio is offering a $133.70 gift certificate to Y-Combinator companies past and present to encourage them to try their in-cloud API for voice communications, which leverages existing web development skills and resources. This gift certificate can help startups build professional voice apps, such as answering calls with a custom 800 number that rings multiple founders at once. Twilio provides an API that allows developers to integrate voice functionality into their products, and the company's developer community hub offers best practices, code samples, and inspiration to build communications and digital engagement experiences.