By Drew FitzGerald

T-Mobile US Inc. is cutting jobs faster than initially planned after its April merger with rival Sprint Corp. created a company with about 80,000 employees.

Rival AT&T Inc., meanwhile, has detailed plans to lay off thousands of workers as it moves through a cost-cutting effort launched late last year.

The coronavirus pandemic that has pressured both companies' bottom lines and changed the way cellphone users shop.

T-Mobile said in a securities filing late Wednesday that it expects to spend about $300 million more than initially projected on merger-related costs, primarily on severance expenses, to accelerate expected cost benefits from the deal. The company now expects merger costs before taxes to total $800 million to $900 million during the June-ended quarter.

The company, which is based in Bellevue, Wash., didn't detail the number of jobs being cut. T-Mobile ended 2019 with 53,000 workers. Sprint last reported 28,500 employees in early 2019.

T-Mobile Chief Executive Mike Sievert said Tuesday the company seeks to hire workers in 5,000 new positions like retail and engineering over the next 12 months. "As part of this process, some employees who hold similar positions are being asked to consider a career change inside the company, and others will be supported in their efforts to find a new position outside the company," Mr. Sievert said.

AT&T has outlined plans to cut more than 3,400 jobs in the coming weeks, according to the Communications Workers of America, which represents a large share of the telecom and media giant's 244,000 employees. Those cuts exclude hundreds of other positions potentially eliminated through store closures.

AT&T said it will make "targeted, but sizable reductions in our workforce across executives, managers and union-represented employees" as it overhauls its employee base. The carrier also is closing more stores to cater to online shoppers, a shift the company said it accelerated in response to the coronavirus crisis.

"Reducing our workforce is a difficult decision that we don't take lightly," AT&T said in a statement.

The savings estimates T-Mobile provided investors suggest several thousand jobs are being eliminated, according to Jonathan Chaplin, a telecom analyst for New Street Research. Those cuts don't include stores run by third-party dealers, some of which will switch to other brands, he added. "They will be cutting redundant positions, but adding other positions as they invest for growth," Mr. Chaplin said.

T-Mobile last year told lawmakers that the then-proposed merger of the two wireless giants would yield more jobs at the combined company by 2024 than each business would employ on its own.

T-Mobile network suffered a nationwide service failure on Monday. Federal regulators said they would investigate the incident, which led to intermittent voice and data coverage for about 12 hours. Company technology chief Neville Ray later said the problems stemmed from a supplier's fiber optic circuit going down, a common event for telecom companies.

Cellphone carriers' network backbones usually have several fallback routes should one path get severed. Mr. Ray said that "redundancy failed us and resulted in an overload situation that was then compounded by other factors." The company said its Sprint customers weren't affected and vowed to put new safeguards in place.

T-Mobile said it expects to close a deal to sell Sprint's Boost Mobile prepaid brand and about 9 million customers to satellite-TV provider Dish Network Corp. on July 1. Justice Department antitrust enforcers demanded the divestiture to ensure the U.S. would keep four nationwide cellular providers. Dish initially will use T-Mobile's infrastructure to serve the Boost accounts until it can transfer them to a new network it plans to build from scratch.

Write to Drew FitzGerald at andrew.fitzgerald@wsj.com