Android
Send a Group Message (SMS/MMS) auf Android
Group messages on Android do not separate SMS and MMS at the moment you write them. The device checks the content once you tap Send and switches to MMS automatically if you attach a photo, a video, or a long text that no longer fits the SMS format.
This automatic switch is the main reason group chats sometimes behave unpredictably. A text-only group message can go out as SMS to everyone, while the same group with an attached image turns into MMS instead, which changes how carriers bill it and how it appears in the thread.
Correct path for a group message
To create a proper group thread instead of separate one-to-one texts, follow this path in the Messages app:
Tap on Messages ➔ Tap on Start chat ➔ Tap on Create group ➔ Select the desired Contacts and tap on Next ➔ Enter a Group name and tap on Done ➔ Choose a SIM card (Dual SIM) ➔ Enter a Text message and attach files (optional) ➔ Tap on Send.
On a dual-SIM device, the SIM selection step decides which number the group sees as the sender and which plan gets billed for the message. If you skip Create group and just add multiple recipients to a normal chat, some phones send individual texts instead of one shared thread.