iMessage and RCS are very similar. iMessage came first, and works with all Apple devices. For the Apple device, you can send an iMessage to a iPhone, or a computer just by knowing the number, or Apple ID. In my case, I choose to disclose my number for iMessage communications. As with RCS, iMessage uses data, to transmit the information and carry conversations to anyone in the world and it would just cost data. 1,000 characters = 1kb. 1,000kb = 1MB, and 1,000MB = 1GB. This means someone sending iMessages will likely not use much data unless they are sending high resolution images. iMessage is automatically set when sending a message to another Apple device. My iMessage number is +1 4126829751