Samsung
Set Default SIM for Text Messages auf Samsung
Samsung phones with two SIM slots default to SIM 1 for every outgoing text, even after you switch calls or mobile data to SIM 2. Messaging has its own default, separate from the SIM used for calls.
Open Settings ➔ Connections ➔ SIM card manager ➔ Messages and choose SIM 1 or SIM 2. The change applies immediately to new texts sent from the Messages app.
This setting matters most if one SIM has a cheaper SMS plan, better reception at home, or is kept for business contacts only. Without adjusting it, texts keep leaving through whichever SIM you set up first, no matter which line you use for calls or data.
On some Samsung models, you can also switch the default SIM for texts from the status bar shortcut, without opening Settings at all. If Messages is missing from SIM card manager, check that both SIM slots are active and that neither one is disabled or restricted by roaming settings.
Switching the default only changes new outgoing texts. Messages already sent or received keep showing whichever SIM actually handled them at the time.