They get your number from other sources..recently they showed Flybuys, some insurance companies, sites you visit on line that want your name & numbers, applications you fill out....there is a bigger resource finger out there pulling the strings. Telemarketers from Pakistan called, telling me about virus in computer, I asked where was he calling from , he said Telecom. Told him don't have computer, he said I registered one, I sent off warranty for new home computer through a Australia wide company which is well known. I asked him what's is my telecom account number, he hung up.This is how they get your numbers.
Spoke to Telecom..firstly they don't call after hours, 2nd they will email or send out mail regarding any issues they have with you. Finally they have all your account details etc on file so don't be fooled by these scammers.
Also when you watching your TV, there may be a advert about cars and different models, a red dot comes up and asks if you want more info on that model, and do you want to test drive. Yes you say, clicking on button, then they say first name last name home or mob number. Yes you say , next they say someone will call you back for your test drive.. They either swap or sell your numbers , who knows ! The link donotcall.gov only reduces the pests it doesn't stop em!