Billionaire Carlos Slim's America Movil announced Sunday that it removed its roaming charges on call and data in the United States for millions of its Mexican clients to prepare for the AT&T Inc, which is trying to penetrate its home turf.

About 40 million Mexican prepaid clients won't have to pay the roaming charges of America Movil to keep up with rival AT&T and stop it from stealing the market share in Mexico. Calls to the United States made through "Amigo Optimo" and "Optimo Plus" plans will now have local rates. This would be the same for customers using data in the US.

The remaining 12 million customers can request to change their plan, according to America Movil. The giant Mexican Telecommunications company is facing a regulatory crackdown in its home country, where 70 percent of its mobile market it.

Since the passing of the law in 2014, AT&T has gained confidence to challenge Slim in his own court by acquiring two Mexican wireless operators, which made operators in Mexico and the US to provide "borderless" call and data plans.

America Movil announced July that it would remove its roaming charges from Mexico to the US for 10.5 million of its post-pay customers who choose to pay 50 pesos extra every month.

These moves to eliminate roaming charges are somehow influenced by the trend among wireless carriers move toward international calling plans. It was wireless giant T-Mobile that first offered free roaming between the US, Canada and Mexico. Sprint followed the T-Mobile with its new Open World SM plan. These companies started these new features in July and August respectively.

But it was the prepaid brands that blazed the trail with AT&T announced January that it would offer unlimited calling from the United States to Mexico for customers under its international calling plan. Boost Mobile plan followed the trend, as well as Cricket Wireless plan.