Vonage's Voice APIs are being used to implement a voice proxy technique that masks the phone number of callers by using an intermediary number, also known as a virtual number. This is done by creating an ASP.NET Core app and using NancyFX to handle HTTP requests, including forwarding calls from Vonage's Voice API. The process involves setting up a Nancy module with a route to answer incoming calls, which returns the NCCO (Nexmo Call Control Object) in response. To test this setup, a new URL is generated using Ngrok and used as the answer_url for the voice application. When a call is made, Vonage's Voice API requests the app to figure out how to respond, and the app responds with the NCCO, effectively masking the original caller's number.