I just spoke with someone with an Indian accent. Really couldn't understand him and I pointed out that his number is captured on websites as a spam. He was unfazed by that. Selling marketing services but his diction was so poor, meaning he spoke quickly and didn't pronounce words fully that I really couldn't understand him.
They call every day, sometimes up to four times. Each time, they leave a "message" only with them breathing loudly for a while.
Very annoying, VERY persistent.
I have found if you put these people on hold they will end up hanging up. I put them on hold and check the number and 99% of the time they hang up before the results pull up. The longer I leave them on hold the less they can call other people also.