When you send text messages on an iPhone with dual SIM, you can choose which SIM card to use. Here's how to do it:
Set a default number: You can select a number as the default number in Settings to call** or send messages to people who aren't in your Contacts app. In addition, you can also choose which number is only used for cellular data. If you want your iPhone to use cellular data on both plans based on coverage and availability, you can turn on "Allow Cellular Data Switching".
Select the number when sending the text message: When you send a message using iMessage or SMS MMS, you can choose which number to use before or after sending.
Open the Messages app.
Tap the "New" button in the top right corner of the screen.
Enter the contact's first name.
Click on the ** number that is currently displayed.
Select the number you want to use.
Switch numbers in a conversation: If you want to switch to another number in the conversation, just tap on the currently used number that appears at the top of the screen and select the number you want to use.
These steps allow you the flexibility to choose which SIM card to use when sending text messages with a dual-SIM iPhone.