How to choose the number of iphone dual-card SMS?

When you start a new conversation and choose the recipient, there will be a button below to prompt you to choose the primary number or the secondary number. You can switch the number of messages sent by this button:

Open the information application, click the "New Information" button in the upper right corner of the screen, enter the contact name, click the current phone number, and then click the number you want to use.

After selecting the primary number or secondary number, the SMS of this number will be sent by the card you choose by default. If you want to change the card, send a message to this number, delete the previous message in the SMS list, restart the session, and re-select the primary and secondary numbers.

If you can't send information through another card:

If you can't switch numbers to make calls or send messages to people who aren't in the Contacts application, please confirm whether you choose to allow this number for iMessage messages. In iOS 13 settings, you can select one number or two numbers at the same time.