I am always of the opinion that a lot of the information comes from a legitimate source, such as a few years ago I had my phone with BT, and despite being ex-directory I constantly got phoney scam like calls from the Far East, coincidentally that's where BT had their call centre' it's the same with email addresses, I have false one which I use for unknown sites who ask for one, and I keep my main one for trusted sites only, even without giving my email address out they still fill up with crap, this is why I don't use the like of Facebook, Instagram etc.they just seem so connected together, and seeing that their income comes from adverts, it's obvious they want your details.
I don’t use my landline for anything, never have done. I’ve checked with BT and they claim they don’t contact by phone to ask for payments and account changes. The number has never been used on sales websites, never been used to make calls, but I get a couple of calls a week telling me I’m about to lose my broadband service if I don’t pay my bill, or I need to make account changes, or my account may have been compromised. These are always calls from a Far East company, I can often hear them talking amongst themselves while waiting for the first multi dialled number to pick up. So, I agree, there’s possibly something corrupt occurring with BT as they’re the only company that actually know my landline number.
On my mobile I used to use an app called Truecaller to alert me to spam calls. It was very effective and allowed me to add any spam caller numbers to their database. Problem was, it put my number and details into a searchable database.
https://www.quora.com/Is-Truecaller-stealing-our-private-data
Not only that, but Truecaller was hacked, giving thousands of phone numbers and details to the highest bidder. Truecaller collects not only your data, required on sign up, but all the numbers in your contacts too. All in the name and good will of protecting you from spam, phishing, etc.
https://alvistor.com/2018/remove-yo...-or-use-proper-privacy-settings/#.XJq2uUmnyhA
Be careful out there folks, I was self employed for many years, but not for the last 10 or so and still get calls about business mobile deals, specialist insurance, fleet vehicle deals etc etc. I’ve had this mobile number for years, it was my first ever mobile number so I kept it. Somewhere, my details are stored and connected to this number....and still being sold.