Anyone looking for a firm in a directory has a concrete need and hence is probably ready to part with their money.
More than 300 million searches are made for (unknown) firms in phone books, phone book CDs, with information services, over the internet or by mobile phone in Switzerland every year. On average, 85% of these queries (across all industries) lead directly to a contact with the firms sought. This makes for around 222 million contacts that result in a purchase or use of a service, or a request for a quote.

So you will be advertising where people are looking for you. There is no more effective way to advertise your services.