Why the mother of your daughter's friend can't call you

This afternoon I received a call for a member of our extended family. I received the call because my number is still listed as a landline in the online phonebook, even though I switched the number to my cellphone years ago. The member of my extended family lives in a household where there is no landline; the parents and teenaged-children have cellphones.
Unlike most of the rest of the world, U.S. cellphone users have to pay for incoming calls. As a result, it has been nearly impossible to get public listings of cellphone numbers. If they're going to pay for the call, people want to control who calls them.
By some estimates, the number of households using traditional phones that connected by copper wires to the phone grid will fall below 10% in five years. A majority of people under 30 don't have home landlines.
How do I reach people for whom I don't have phone numbers? Often, I have an email address or a Facebook, Twitter, or LinkedIn connection. In more than a few cases, I have a secondary connection, such as a cousin-in-law, on one of the social networks. It might take a few detective steps, but I think I can make contact with most of my family and friends.
Phonebooks have become urban litter because we have shown that online information is better and more readily available. An interesting unintended consequence of this always-connected world, however, is that we've lost a way to find the phone number of the parents of your daughter's friend.