Sprint and its largest shareholder SoftBank have started preliminary conversations to merge with T-Mobile US, Bloomberg reported on Friday, citing people familiar with the matter.

The U.S. Federal Communications Commission had barred merger talks among telecommunications companies for more than a year as it conducted a $19.8 billion auction of airwaves from broadcasters for wireless use.

Reuters reported in February that SoftBank was positioning itself for deal talks with Deutsche Telekom, which owns 65 percent of T-Mobile, once the auction was out of the way.

SoftBank Chairman Masayoshi Son and Deutsche Telekom Chief Executive Officer Tim Hoettges separately told investors this week they'd be open to discussions about consolidation.

A Sprint merger with T-Mobile comes fraught with regulatory risk. Sprint had been interested in acquiring T-Mobile in 2014, though the deal fell apart on regulatory concerns.

T-Mobile is larger of Sprint, which has struggled to retain customers even after offering the industry's cheapest unlimited data plans. Sprint is languishing in fourth place in the U.S. wireless market.