Every phone will have an SMS App which is used to send SMS messages. If the message is longer than the 160 character limit the App will split the message and send using the Long or Concatenated SMS protocol. If the user tries to introduce multi-media into the message the default app will use the MMS protocols to send the data. When receiving an SMS message, the App will try to be helpful to the recipient by highlighting links, emails, phone numbers making them clickable. This is done within the App without actual links in the SMS message.

On an Android phone both Samsung Messages and Google Messages have extended the features of the basic messaging systems to include RCS. This allows a chat system over and above the basic messaging and allows groups, read receipts, multimedia content. RCS can work between any two devices that support RCS and defailts to SMS / MMS when not available.

On an Apple phone they have iMessage. This is a chat system similar in nature to RCS but restricted to Apple to Apple devices. Any messages to an Android phone will (now) be sent by RCS, which defaults to SMS / MMS on any phone not using RCS

When a basic SMS message is sent, it does so over the control channel. Thus the basic SMS message can be sent when there is limited data connection. However any multi-media content requires the data network

The iMessage and the Google RCS infrastructure are separate systems, and using a chat system between the two does not work well. In the US where most phones are Apple this is not an issue, but in the UK where there is roughly a 50/50 split iPhone vs Android the messaging systems get in the way of a unified messaging system. As an alternative there are a number of messaging applications for which Whatsapp is the most popular. This is build in a similar manner to the older chat systems using the XMPP protocol. The user is presented with a screen not disimilar to an SMS app where they choose recipients, add text and multimedia. The message is sent to one of a number of servers. Whereas SMS, and it's derivatives work over the phone system, the contriol channel, Whatsapp uses HTML Web Sockets. The WebSocket send() method is used to send the message. The onmessage() method is used by a user to see if there is any messages for them and if so these are downloaded. Note that Whatsapp and other App based messaging systems use Data sevices HTTPS and the Internet and not the telephony control system that SMS uses.