Many folks don't know that when you book airline tickets through a travel agency the agency does not share your contact information with the air carrier. This includes the online agencies. Why is this a negative, you ask?
Let's say you've booked far in advance. Airline schedules change periodically. In this example you've purchased a ticket on the 10pm flight. For whatever reason - airport curfew changes for instance - that flight now departs at 8pm, but without your contact information the airline can't notify you. The only number the carrier has is of your agent, who is notified, but more often than not, does not pass that information on to you. While one should always confirm their flight details the day before, many folks don't. That results in the passenger showing up at the airport and finding out they've missed their flight. And if it is the last flight of the night....well, you get the picture.
Please, if you've purchased air transportation through a travel agent, call the carrier and have them add your phone number to the record.