We still have a landline because my aunt prefers it to her cell phone. She recently decreased the bill by getting rid of features she thought we didn't need, like Caller ID. So now when the phone rings, we have to answer it. If it's a live person trying to sell me something I politely but firmly say, "take my name and phone number off your list." They are then obligated to do so, I've been told. In any case, it usually works.
On my cell, I just decline the call if I don't recognize the number. Almost nobody has my cell number. I don't give it out except to family and a few friends and my psychiatrist's office. If there's a number space to be filled in online, I put in the landline number.