iOS 18 RCS Fallback Options Explained

With the release of iOS 18, Apple introduced Rich Communication Services (RCS) to modernize messaging between iPhone and Android users, but not all carriers currently support the standard. When an RCS message cannot be routed due to an unsupported carrier or network limitation, the operating system uses specific fallback mechanisms to ensure message delivery. This guide details how iOS 18 handles communication when encountering non-RCS carrier networks, how fallback options function, and the settings required to manage them.

Automatic Fallback to SMS and MMS

The primary fallback option in iOS 18 when communicating with a non-RCS supported carrier is standard cellular messaging via SMS (Short Message Service) and MMS (Multimedia Messaging Service).

When a user sends a message to a contact on a carrier network that lacks RCS Universal Profile support, iOS 18 detects the absence of active RCS endpoints during network handshakes. Instead of failing the transmission, the Messages app automatically routes the communication through legacy cellular channels:

  • Text-only messages automatically convert to standard SMS messages.
  • Rich media, photos, audio clips, and long texts automatically convert to MMS messages.
  • Group chats containing one or more participants on unsupported carriers default entirely to MMS group threads, forfeiting RCS features like high-resolution media sharing, typing indicators, and read receipts.

In all fallback scenarios, the message bubbles will appear green rather than blue (iMessage), and the text field will indicate "Text Message" rather than "RCS Message."

The "Send as SMS" Setting

The fallback mechanism relies directly on system-level settings within iOS. To allow automatic fallback when RCS fails or is unsupported:

  1. Open Settings on the iPhone.
  2. Tap Messages.
  3. Locate and toggle Send as SMS to the On position.

If Send as SMS is enabled, iOS 18 automatically falls back to carrier SMS/MMS without requiring user intervention whenever an RCS transmission fails or cannot establish a connection.

If this setting is turned off and the recipient's carrier does not support RCS, the message may display a red exclamation mark and a "Not Delivered" status. In this case, the user must manually tap the message and select Send as Text Message to initiate a one-time SMS fallback.

Carrier Configuration Dependencies

RCS in iOS 18 is not globally active by default for all networks; it requires an updated Carrier Bundle provided by the specific mobile operator. If the recipient's carrier—or the sender's carrier—has not yet deployed carrier-level support approved by Apple, the Messages app will not attempt an RCS handshake at all. Instead, it bypasses RCS completely and initiates the thread as standard SMS/MMS from the start, preventing delivery delays.