They get your cell number by having a computer dial a pattern of numbers and they sequentially eliminate dead numbers or active numbers. Get a phone call with no one there and it hangs up? You just told their computer there is somebody at that number. Sometimes they're just calling to create a list of active numbers that they can sell, other times the computer is supposed to transfer you to a telemarketer, but no one is available, so it hangs up and flags your number for a call back (which of course will hang up on you again if no telemarketer is available. What an enjoyable loop of harassment that is).
When I was in high school I answered an ad that promised nearly four times minimum wage, it turned out to be a telemarketing job and the wage in the ad was based on a near impossible to reach quota. This was before computers did the dialing, so they gave us a piece of paper with a partial phone number on it (eg. 613-555-50XX) and on the rest of the sheet was a grid from 00 to 99. We would dial the partial number and combine it with the number on the grid (eg.613-555-5000, 613-555-5001) and indicate if someone answered the phone (in which case we tried to sell them some coupons), an answering machine (which was marked for a call back), business, or inactive number. Using this grid you would also end up calling unlisted numbers, which really pissed people off. That was my first and only shift in the telemarketing industry. What a [***] job.
It was shady then and it is shadier now with VOIP numbers and Caller ID spoofing. I add numbers to my block list (*60 on a Shaw Phone), but eventually you run out of slots (32 is the max). I also created a contact called "Don't Answer" on my cell phone and assigned it a silent ring tone, adding numbers I don't recognize where a Google search brings me to a page like this one. Next time they call, it won't disturb me, I'll just notice a missed call from "Don't Answer" next time I pick up my phone. But still, I shouldn't have to do all that. Most people I know don't answer their phones unless they recognize the number because the harassment from scammers is becoming a daily ordeal.
The government should bring some accountability to the VOIP providers and telephone companies, but a doubt any politician has the balls.