The pertinent line is:

As he went he thought he heard a far-away voice that he once knew call “Ralph!” 

The voice can only be that of Annabel Adams. Jimmy Valentine feels that he has lost everything by exposing himself as a top professional safecracker in order to save the life of a little girl trapped inside the bank vault. The effect of the "far-away" voice calling him by his adopted name is to show that Annabel still loves him but that he is certain he has lost her. The story ends without any reunion between Jimmy Valentine (Ralph Spencer) and Annabel. O. Henry realizes that there would be a great deal of questioning and explaining to be done after Ben Price allowed Jimmy to "retrieve" his reformation. That would have been like a whole different story--but the single word "Ralph" which Jimmy hears on his way out of the bank is intended to assure the reader that such a "retrieval" of the girl he thought he had lost and of the respectability he thought he had lost with her, was possible.

We are left at the end of the story knowing that things will work themselves out. Because of "Ralph's" heroic action and personal sacrifice, Annabel will understand and forgive him, but only after he makes a full confession. And if Annabel understands and forgives him, then her whole family will do the same. Jimmy will be able to retain his new identity as Ralph Spencer, businessman, family man, respectable citizen, pillar of the community.

The real turning point in the story comes when Ben Price shows he has accepted Jimmy's reformation.

“Guess you're mistaken, Mr. Spencer,” he said. “Don't believe I recognize you. Your buggy's waiting for you, ain't it?”

And Ben Price turned and strolled down the street.

This is a good ending. If Ben Price can forgive Jimmy, then there should be no serious obstacle to being reinstated with Annabel and her extended family. Anything more at the end would be anticlimactic. The single word "Ralph!" which Jimmy imagines he hears on his way out of the bank is sufficient to assure the reader that that part of the problem will be resolved. Jimmy has only partially "retrieved" his reformation through the generosity of Ben Price. The title of O. Henry's story assures us he will retrieve it completely in the unrevealed but inevitable denouement.