Notwithstanding my often garbled posts on here, I work in communications and do a lot of writing and editing.
From a sheer sales perspective - calling people in the yellow pages may well lead to some work on the basis that any cold-calling is always a numbers game: call enough people, someone is going to bite. But I think you'd have to make a LOT Of calls to get any results.
However, you should know that copywriters are often not valued. They will be shocked you are charging them to do something they believe they can do themselves. How will you pitch yourself? Why should they hire you?
Also, what exactly do you see as copywriting requirements - are you going to rewrite their website? Offer to craft their social media posts? Write blog posts? What, if anything, do you know about their market/sector? What, if any, experience in copywriting can you post to?
If you want to become a copywriter, it may be worth signing up to some sort of website where you can get small jobs and some experience. Or ask around friends/family if you can do work for them at a low fee to have something you can point to as having been effective.
Certainly, I employ many freelancers but I wouldn't dream of employing someone who couldn't produce evidence of their ability to write and to translate what the client wants said in terms of content, tone and style.