Samsung
View Messages Stored on the SIM Card auf Samsung
On current Samsung phones, the SIM card is no longer used to store new SMS messages by default. If you switched a SIM from an older phone, or a carrier stored texts on it earlier, those messages still exist on the card but sit outside the normal inbox view.
The Samsung Messages app keeps a separate viewer for this. You will not find these texts by scrolling through your regular conversations, since the app treats SIM storage and device storage as two different message pools.
To open it, go to Tap on Messages ➔ Open the Menu ➔ Tap on Settings ➔ Tap on More settings ➔ Tap on Text messages ➔ Tap on View messages on SIM card.
This screen only displays messages already stored on the SIM. It will not let you save new incoming texts there, since that function was removed from modern Android messaging on both One UI and stock Android builds.
If the SIM card option looks empty, the card likely never held any messages, or a previous factory reset cleared them. In that case, check your regular Messages inbox or a backup app instead, since the SIM viewer only reflects what is physically written on the chip.

