Sprint (NYSE: S) is the only U.S. carrier to publicly back Google's (NASDAQ: GOOG) recent move to create a Rich Communication Services (RCS) client for Android, but T-Mobile (NYSE:TMUS) continues to gain traction with its own RCS service.

The future of the messaging standard is as uncertain as ever, however.

T-Mobile eight months ago became the first major U.S. operator to offer RCS, rolling out the service under the brand Advanced Messaging. Today, more than 5.5 million T-Mobile subscribers now use the service, the carrier told FierceWireless, sending 40 million messages per day. Advanced Messaging is available on 10 T-Mobile devices, eight of which support video calling via RCS.

T-Mobile's Advanced Messaging also supports sharing of videos of up to 10 MB in the phone's native dialer "out of the box" rather than through an OTT app. The carrier said nearly 6 million devices support its RCS service.

RCS -- which was once dubbed joyn -- is a messaging standard that was developed as a way for mobile operators to better compete with services such as WhatsApp, Facebook Messenger and Apple's iMessage, which have eaten into carriers' SMS revenues around the world. RCS supports features such as group chat and high-resolution photo sharing that aren't available through SMS.

The GSMA announced last week that Google will develop a universal RCS client for Android, signaling the association's continued effort to build momentum behind the messaging standard. But other operators haven't enjoyed the success T-Mobile has seen with the alternative to SMS, and the long-term viability of RCS is in doubt. Indeed, ZDNet just this month reported that two of South Korea's largest wireless carriers -- KT and LG Uplus -- said they will discontinue their support for RCS, largely due to the popularity of the KakaoTalk messaging service in that country.

Whether Google's support can give RCS the boost it desperately needs is unclear, however.

"In our view, this is the GSMA's last hope for RCS," CCS Insight wrote. "If Google is able to establish its RCS client as a truly universal and usable client on Android devices globally, it may offer a way to reduce Facebook's stranglehold on messaging services…. We're not sure RCS-based services can compete against well-established and well-liked proprietary apps. It may be too late to joyn the GSMA's initiative."

For more:

- read this CCS Insight post

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