I think the message delivered is very valid, but the delivery was pretty rough around the edges. Never give your work away, because you can always negotiate for something in return. In this case, a business card size add, in bold, red, etc, would be a reasonable 'something' vs 'nothing'.
I've had magazines and compaines call to ask to use an image, and they usually try to convince me that I already shot the image, so the cost to allow them to use it is nill. In a very polite way I just counter with asking them to run a full page ad for my business since the ink can't cost all that much either. The value of both usually comes across in the ensuing exchange 
The point is that there is usually a way to get something in return. Something that would be easy for the buyer to give, and something that is of value to the photographer. But any working photographer will tell you that a 'credit' is of little value 99% of the time. If they can't offer anything at all, then run, don't walk.
The fomula I use to determine if a trade is worthwile to me is to assign a normal value to my work, then think about whether or not I would buy what they are offering for that value. In this case, would I pay $2,000 of my advertising budget to have my photo on the front of the phone book with a credit line? If the answer is yes, its probably a reasonable trade. If not, I would need to negoitate for more.
On the other side of the token, if Sprint would just move on to the next person willing to submit the photos for free, then the $2,000 value is a hollow value to begin with. The cover of a Sprint phone book would no longer be worth $2,000, but more like Free to maybe a few hundred at most. IMO, the OP didn't loose $2,000, because that figure would never be on the table to begin with. Maybe 10 years ago, but not today. Thats the effect of lots of people willing to give away their work, microstock, etc, and how it devalued the market. Thats a perfect example of how people giving away images have take work away from the working photographers. Phone book covers used to bring in a decent pay rate 10 years ago. With microstock, a phone book cover is now about .20 a download, or free!
Yes, it is cool to be published. And congrats to the OP. In this case the OP can probably still use this to their advantage. Ie, reproduce the cover and use it as part of a promo peice. Submit a press release to the local papers whooping it up that Sprint as selected her image for the cover of their phone books. There are plenty of ways to put a positive spin on it and still get lots of value out of it.
But also take it as a lesson learned. In the future, negotiate for as much value as you can get. Ignore credit lines, as they really are worthless based on every pro I've spoken to. If a client offered me a credit line for my images in exchange or $15 off my regular invoice price, I'd decline and take the cash instead. That puts a value on a credit line for me right up there with .20 a download 