Joyn is so far available only in Spain, although it is due to be launched in Germany in October and South Korean operators and others are expected to start RCS-e services soon.

ECONOMIST: Over-the-top phone services

The downloadable client helps bridge the interoperability gap and address the fundamental challenge of mass adoption by enabling carriers that have yet to deploy RCS to offer joyn's enriched features to their subscribers.

ENGADGET: MetroPCS widens Rich Communications Services to all North American carriers

Last year, MetroPCS became the first U.S. mobile operator to offer RCS 5.0 services on a 4G LTE network under the joyn brand and has now joined leading global operators in receiving provisional GSMA joyn accreditation.

ENGADGET: MetroPCS widens Rich Communications Services to all North American carriers

Right now, the very young state of RCS on LTE leaves it behaving more like the isolated services it's trying to replace -- on MetroPCS, only those with the Galaxy Attain 4G and an after-the-fact Joyn app download can get the experience as intended.

ENGADGET: MetroPCS intros first Rich Communication Services on LTE, touts universal contacts and chat (video)

When consumers see the joyn brand they will know that the device is RCS enabled, which means it automatically 'knows and shows' ways to share with contacts via chat, video, call or files, at any moment in time, regardless of the network or device used.

ENGADGET: MetroPCS widens Rich Communications Services to all North American carriers