In some cases although you need a landing page users wont see it as some people are using adwords paypercall only ads to drive targeted traffic to the phone number. Since the ads are very limited in space the url is an important aspect. Others use display ads, etc. Most programs only allow online marketing, although their are exceptions.
Ringpartner for example has many programs from find a lawyer to home services such as carpets and flooring from known brands such as empire which pay $20+ plus a qualified call.
In the paypercall market industries that rely on calls such as plumbers, locksmiths, etc. Are the best fit since the customer will not need more info before placing the call.
Each program has hurdles such as the user having to press #1, or #2, etc. As well as be on the phone for a certain time before the call is considered qualified.
P.s. Paypercall is mostly for mobile users since they are able to make the call directly off their phone.
P.s. 2 if you have existing traffic related to an offering you should place a ppc number on your site and try it. You have nothing to lose but a little bit of your time.