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Hey there and welcome to Apple Support Communities MaggieFeng,

We can certainly understand your concern and see you came to us from the article What is the difference between iMessage and SMS/MMS?.

We recommend checking out these steps for more information on how to turn off iMessage in order to just use SMS and MMS via your carrier. You can locate the setting by going to Settings > Messages > iMessages and toggle the feature off. If you wish to reactivate the feature, please follow the same steps or you can check out the link below for more details:

Set up Messages on iPhone - Apple Support

These steps will help ensure your device is working with ease and can fix common issues like the ones you’re experiencing.

Kindest regards!