There are basically 4 categories of telephone numbers
Out of service
Rings no answer
Rings and answers, artificial voice recording
Rings and answers, human voice recording
Rings and answers, human real time voice
Computer's dial the numbers, all numbers, its not "random", Cell or Landline doesn't matter
That number is listed as one of the above, with time of day
Lists are sold or traded between services that want access to them
Numbers are called repeatedly over time to see if status changes, i.e. back in service, time of day answers, if real or artificial answering
Its all just data to sell or trade
Certain area codes may be targeted over a few weeks then move on to another area code
Real time human answering numbers are worth the most, even if hostile.
So if you answer a call from a marketing company you will get other calls from that marketing company and others when they sell your number to them, you are the golden goose, lol, to them
And yes they change caller IDs/numbers to avoid being blocked
Seniors are often targeted in this manner, and many do still use Landlines, so yes these numbers can have more spam calls hoping for seniors to answer
Seniors tend to be more polite and because they have lived through so many changes in technology and feel they are out of step with current "things", so if some one calls claiming to be from the government, or credit card, or bank they may take them at their word and give out information they shouldn't
If you want to stop spam calls don't have a phone, literally the only way
Less spam calls, never answer if you don't know the number calling
Spam blockers use caller IDs from known exchanges for spam calls, they use data to connect to APP servers to check the caller ID before its allowed to ring thru to your phone