Actually, it is all based upon algorithms of potential valid phone numbers. You don't have to give your number out to anyone to get these calls. Automated dialing systems call thousands of these numbers randomly on the off chance it is a valid number or someone answers. That is why so often if someone does answer, there is no one on the other end of the line. The system just records the fact that someone answered so the number is entered into the system as a valid number. Often these numbers are compiled into lists and sold to telemarketers as call lists.
An electrical outage that lasts more than 8-10 hours means yoru cell phone & laptop will be dead. If you don't have a car & a car charger, you'll have no phone service without a landline. That's why I keep mine.