8x8 is a company who specializes in services to fit the needs of businesses in the areas of voice, video, mobile, and unified communications. They offer a suite of APIs to couple your needs with the Virtual Contact Center offerings of 8x8.
"Web callback" allows for embedded buttons on websites allowing visitors to request a 'callback' from the application. Utilizing callbacks, customers do not have to wait in the support queue and can interact directly with the application.
Features of the API include: call me now, call me after a certain delay, call me when agent is available. All callback requests (GET, POST) are made using HTTPS.