Both
I do it myself and I pay companies like superpages and Yp.com to do it as well.
When you pay those type companies, you get a link to your website from huge reputable companies and Google scrapes them to help your website ratings in the organics.
When you get on their sites, you get on all their other sister type sites as well.
It pays to be on them for more than just the listing they give you.
You can sign up with Superpages and not have a 12 month contract if you do it yourself online. They have a guarantee click program.
YP.com want that contract from what I can tell...no matter what.
Kudzu is a 3 month minimum contract
With anything, you need to track where you clicks are coming from.
If your phone calls don't slow after canceling, it doesn't mean the ppc wasn't working. Some things ...most things...linger around the internet until they fall off.
It's an ongoing process to keep your name out there. Once you let it go, it takes some time to bring back.
Consistency and Variety