Wirecutter – The research:” When people talk about spam calls, they tend to refer to four types of calls:

• Telemarketing: These calls originate from a live person who is trying to sell you something from a legally registered business. Such calls are annoying but generally not fraudulent.
• Legal robocalls: Legal robocalls are automated calls for notifications, services, or sometimes bills. A robocall is not inherently spam. Everyone gets legitimate robocalls from doctor’s offices confirming appointments, political campaigns looking for votes, debt collectors pursuing money, charities contacting prior donors, or schools making announcements.
• Illegal robocalls: Illegal robocalls include many prerecorded messages you didn’t sign up to receive. This category includes calls such as a sketchy auto-warranty call, student-loan scams, or a call that went out during the presidential election telling people not to vote (subscription required to read article). In some cases, such as the auto-warranty call, you’re asked to press a button to connect to a live person, who then attempts to scam you.
• Scam calls: When a live person calls and tries to defraud you in some way, that’s a scam call. This category includes everything from the aforementioned car-warranty scam to kidnapping scams. There are so many robocall scam variations that the Federal Communications Commission keeps a glossary of different iterations…”