You would be amazed how many people call in, ask to do a courtesy hold, call back the next day only to be stunned that the fare has gone up X amount of dollars. (One person called me every name in the book when I had to inform her that the $210 last minute fare from RDU-PHX had, in fact, been cancelled earlier that day and the current fare was $986.) Every res agent is trained to ask for a credit card to secure the fare and inventory space. As a matter of fact, when people ask me to do a courtesy hold, I remind them usually at least twice the fare is not guaranteed without a credit card (or if more than ten days out, they can mail a check to NW and guarantee the fare). On top of that, most, if not all, agents will document your reservation if you decline to give a form of payment, so saying "I wasn't told" never works.
I do understand how it can be frustrating for pax, as different airlines have different policies. I think AA, as an exmaple, will hold the fare for you, but once you give a form of payment, it's immediately non-refundable. NW does the opposite.
Other note: giving a CC over the phone, while it does not place an immediate charge on your card (which would then show up on your billing statement), it does do an authorization, thus locking the funds away from being used by you. It can usually take four or five days to release this hold on your money (or restore it back to your available credit).
Oh, and most definitely call reservations. Each call that comes in is just a little more proof that they need to keep
some of us employed!!

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