iMessage (+1 4126829751) is based off of Apple’s closed IM system. This platform does everything RCS does, but between Apple devices. There is also FaceTime communication which is as well – closed loop. I am OK with this as iMessage is Apple’s proprietary capabilities. If they were to not support SMS, then I would have a problem with that. iMessages uses the Messages application on both Mac PCs, and iOS devices. Selecting that link on a device not an iPhone will bring up the default SMS client on your phone.