How Do Robocalls Work?
A robocall delivers an automated or prerecorded voice message. Unless the use of a particular number was permitted, the call is illegal in many countries. In the US, the Telephone Consumer Protection Act and the Telemarketing Sales Rule forbid companies from using an automated dialer to call or text a phone without your permission. Most illegal calls are also spoofed, meaning that the caller ID is falsified to look familiar to the recipient.
In most cases, scammers buy lists of hundreds of thousands of numbers from brokers. They then feed the numbers into a computer program that operates in a loop, dialing through the list and attaching a spoofed number to each call so that it looks like the call is coming from a local number. This type of software, which is fairly simple technologically, can make as many as a million calls in an hour.