“The answer to that question,” he said, “is plainly no.”

But Justice Elena Kagan said there was something curious about the case. “It doesn’t make much sense, though, does it?” she asked Mr. Shanmugam.

“Your client serviced this debt and counted as a debt collector at that time,” she said. “And then your client purchased the debt and all of a sudden is not a debt collector.”

“And I guess the question is: What happened in between the time when your client serviced the debt and the time when your client purchased the debt that in any way changed its relationship with the borrower such that Congress wouldn’t be concerned any longer with its behavior?” she asked

Mr. Shanmugam said his client, as owner of the debt, had different interests than some debt collectors, including keeping a good relationship with customers to whom it might want to sell other products. “I do think,” he said, “that we would have incentives to maintain their good will in the way that the sort of fly-by-night debt collectors that Congress was seeking to target 40 years ago” did not when it enacted the law.

Kevin K. Russell, a lawyer for the borrowers, faced resistance even from justices who might be inclined to be sympathetic to his position. “I can never get it to mean what you want it to mean,” Justice Kagan said of the words of the law.

Mr. Russell said his reading of the law avoided some contradictions and superfluous words. But Justice Samuel A. Alito Jr. said the supposed inconsistencies were inconsequential. “You’re really going uphill on that,” he said.

Mr. Russell responded that some of the language in the statute created “a null set” or “Venn diagrams with circles that do not touch.” He gave what he said was a comparable example: “If Congress has enacted a statute that regulates taxis only to the extent they’re driven by poodles.”