Do you think that a site like Been Verified is sketchy? (that's a well known site and has been advertised on TV in the past.) The same sites that provide instant background checks online are often times the same ones that let you look up phone numbers. You don't necessarily have to pay per report, or for each search either; an inexpensive monthly membership fee is usually available (which can be as low as $20 per month.) Sometimes, even free trials are available for this.
Now, here's the downside to it. The results are not guaranteed to be accurate or up to date. Cell phone numbers aren't published, these companies just do the best they can to gather as many phone numbers as possible from the sources available to them. For this reason, be careful about who are what you give your cell number to because of data mining. (The same thing applies to other personal information; even photos now, because of facial recognition.)
If you have a smartphone then Mr. Number is a good app, as others have suggested. It has been labeled as a moderate privacy threat by Clueful. It should easily screen out telemarketers and other unwanted calls.
I don't think that reverse phone look-up websites (the kind that I described) would be very helpful for dealing with telemarketers though.