iMessage is a proprietary Apple Communication standard that predates RCS?, and was designed to provide a rich communication standards those using Apple devices. With iMessage, one could simply send a message to another Apple user by using the device’s data connection to the internet. This means a desktop computer can use the wired connection to send an iMessage to a cellular phone while the phone is using the cellular network. The only thing that is needed to be known is either the Apple ID (an email address), or the associated telephone number(s) with the account. There is nothing special that needs to be done. Apple keeps database of every telephone number that is used as an Apple ID, and will know when to send an iMessage. Linking a phone number on a webpage (like above) will use the SMS? link protocol.