(CBS News) When it comes to booking flights online, what you see is not always what is available. Airlines routinely hold 30 to 40 percent of their seats, which means that flights that look sold out online could still have multiple seats available.
Since flights are appearing fuller when making online reservations than they actually are, customers are feeling pressure to pay extra to guarantee a premier seat. Online shoppers typically only see a selection of middle seats -- which is typically a red flag for anyone traveling with a friend or family member so they choose to spend the extra bucks to be seated together.
According to the Wall Street Journal, every airline has a different definition for what exactly is withheld and how those seats get opened up as people buy or don't buy tickets. There is no industry guideline and each airline only has to make available online what they want to.
CBS News travel editor Peter Greenberg told "CBS This Morning" that the airlines are doing this in an attempt to get people to pay the extra money for a "premium" seat - which often translates to anything but a middle seat.
Greenberg explained that some of the airlines hold seats for airport check-in as well as for elite customers, so the seat you paid more for could have been available anyway when you arrive for your flight.
He said that the best way to circumvent these seat blackouts and avoid frustration is to call the airline directly to see which seats are really available and, "Do not depend on the Internet to be your friend."
Greenberg said it can be easy enough to get someone on the phone, though it's helpful to call at the right time.
"It depends on when you call. If you call at five in the afternoon or at eight o'clock in the morning, forget about it," said Greenberg. "Call at midnight. And then never take a 'no' from someone who's not in power to give you a 'yes' in the first place." If needed, ask to speak to a supervisor.
For Greenberg's full interview, watch the video in the player above.