AT&T is adopting a new roaming scheme that offers free voice and data roaming in Canada and Mexico starting on May 20, but it doesn't apply to all customers.

The new treats are available only to select AT&T subscribers. More specifically, only some postpaid customers will be able to benefit from the free roaming upgrade, as follows.

Customers of AT&T Mobile Share Value plans with a quota of at least 15 GB of data per month will get free voice, text and data roaming in Mexico. Enterprise customers, meanwhile, will get the free roaming feature if they are on Mobile Select plans amounting to at least $20 per month.

AT&T subscribers on the carrier's new unlimited data plan, which is only for DirecTV and U-Verse customers, will get to enjoy free voice, text and data roaming in both Mexico and Canada.

"We know our customers want the same experience abroad they're used to domestically. And only AT&T can deliver the premier LTE experience they want," says AT&T's Chief Marketing Officer, David Christopher.

According to Christopher, roughly 20 percent of AT&T postpaid customers take trips to Canada or Mexico annually. Moreover, AT&T wants to boost 4G LTE coverage to 100 million people in Mexico by the end of 2018.

To activate the new roaming upgrade, customers on AT&T Unlimited Plans will have to add the AT&T Roam North America feature separately to each smartphone, and the Roam North America Data feature to each tablet. Adding these involves no extra cost. Customers can add the features through myAT&T, online on the carrier's website, or by calling AT&T.

AT&T also tweaked its prepaid GoPhone offer back in February 2015, throwing in 4 GB of data and free calls to Mexico for $60 per month.

With the new roaming scheme, it's worth pointing out that AT&T is also taking some measures to prevent abuse. On this note, the carrier says it may limit the feature if roaming usage reaches more than half of the total usage for two months in a row.

It's still a good deal and should make some subscribers happier with their service. At the same time, AT&T's roaming upgrade for select subscribers doesn't really rise up to T-Mobile's Mobile Without Borders, which offers free voice, text and LTE data to most of its postpaid customers.