T-Mobile may be missing out a huge revenue stream, as it offers customers free international roaming. 

A new report from Juniper Research estimates operator revenues generated from mobile data roaming at $42 billion by 2018. According to the report, that represents 47 percent of the global mobile roaming revenue, which includes voice roaming, compared to an estimated 36 percent in 2013.

As might be expected, that increase is driven by the continued expansion of LTE networks around the globe. Reductions in roaming charges may also spur more frequent, heavier usage, according to Juniper.

Key to capitalizing on roaming revenues lies in carrier agreements. One of the major obstacles to those agreements are the many different frequencies at which LTE is being deployed. 

Report author Nitin Bhas said Operators also need to sort out the right economics to encourage more usage at a value to the end users in order to avoid revenue erosion. 

“They need to also provide services that are both relevant and cost effective to LTE roamers,” Bhas said. 

The report comes as the European Union works to end roaming charges between countries. Juniper takes this into consideration, saying that should the EU get the European Parliament approval to end roaming charges it would “significantly impact on roaming revenue levels.”

Under this scenario, the report forecasts that European voice, SMS and data revenues would decline by just over 20% in 2016. However, the actual volume of usage and number of active roamers will continue to rise over the forecast period.