In order to find potential customers which is better : In person or by phone

My gut feeling tells me that nothing beats the "in person" approach. But just in case is there an occasion where approach by phone would be better?

Let me give you examples of probable approach.

First approach : Better in person.
Other approaches (in case first approach potential customer was not interested): Always preferable in person?

How long should we wait before contacting again a potential customer that was'nt interested the first time we visited them?