When a teenage boy snatched the iPhone out of Rose Cha’s hand at a bus stop in the Bronx in March, she reported the theft to her carrier and to the police — just as she had done two other times when she was the victim of cellphone theft. Again, the police said they could not help her.

Ms. Cha’s phone was entered in a new nationwide database for stolen cellphones, which tracks a phone’s unique identifying number to prevent it from being activated, theoretically discouraging thefts. But police officials say the database has not helped stanch the ever-rising numbers of phone thefts, in part because many stolen phones end up overseas, out of the database’s reach, and in part because the identifiers are easily modified.

Some law enforcement authorities, though, say there is a bigger issue — that carriers and handset makers have little incentive to fix the problem.

“The carriers are not innocent in this whole game. They are making profit off this,” said Cathy L. Lanier, chief of the police department of the District of Columbia, where a record 1,829 cellphones were taken in robberies last year.