The text discusses the use of Bandwidth's Voice API and BXML to build Interactive Voice Response (IVR) systems. It explains how IVR systems can be built using BXML, a custom XML language that defines operations for phone calls on Bandwidth’s network. The author provides an example of building an IVR system with BXML for a plumbing service's customer satisfaction survey. The text also highlights the differences between Bandwidth's Voice API and BXML in updating phone calls and retrieving call information from Bandwidth.