They are criminals. The calls are illegal. That is why they don't leave messages. That is why the number(s) is invalid. They want you to act on impulse and provide your personal information like social and credit card.
NEER EVER provide personal information over the phone. Not even name and address. Be smart folks. These scams persist because gullible people continue to fall for them. Telemarketing scams are becoming like email spams.