iPhone RCS: Must be an AT&T wireless customer with an iPhone running iOS 18 or later with RCS messaging enabled, on an eligible data plan. Does not work on Watch, iPad or other mobile iOS devices. If data is blocked on your line, RCS will not work. 
International: RCS outside our network coverage area should work using Wi-Fi data. RCS for iPhone may not work in all overseas destinations.
Charges: Each message, chat or file attachment sent via RCS (or iMessage) counts as data and is charged as data. If sent as MMS or SMS, each message or file attachment will count and reduce your messaging allotment. Content/messages sent via RCS can be larger than MMS and may use large amount of data.
Restrictions: Maximum file size may be dependent on the device, firmware version, and current network conditions. AT&T reserves the right to change the message size limit at any time without notice. If you’re experiencing slow speeds due to congestion, restrictions on your plan, roaming restrictions or other factors, RCS messaging may be impacted and may not work.