Straight talk Customer Service saying they are sprint phones?

[pawncentral]

I buy used straight talk phones to resell in my pawnshop...I am careful to always ensure the ESN's are free and clear to be activated by new customers, but in the last 2 days I've had two phones returned because the CS reps say they are sprint phones. Sometimes the customer actually has to call straight talk 2 or 3 times before they actually free up the phone for use. I have never had this problem before? Any ideas? What's the deal?!