The move by Verizon comes as consumers and lawmakers are increasingly concerned about privacy and security amid data breaches by tech firms, including Facebook Inc..

In a letter to Senator Ron Wyden of Oregon dated June 15 and released by Wyden’s office on Tuesday, Verizon said it was beginning the process to stop selling customer location data to vendors that aggregate the data.

Wyden contacted the major carriers after his probe found that a prison phone company called Securus Technologies with access to such data had allowed law enforcement to use it to track people.

A Securus spokesman said the company was authorized to give law enforcement the location of a phone in certain circumstances, under Securus’ contract with the third party data aggregator.

“We believe that ending the ability of law enforcement to use these critical tools will hurt public safety and put Americans at risk,” the spokesman said.

AT&T Inc and T-Mobile US Inc said in letters to Wyden that they have blocked the prison phone company from accessing customer data, but stopped short of saying they would stop selling the location data to others. Sprint Corp in its letter to Wyden said it would end access to its customers’ location data if a breach was found.

Shares of Verizon were up 2.2 percent at $48.49 in afternoon trading.