How does iphone switch the cards that send messages?

1. Open Info, and then click the edited small icon in the upper right corner. ?

2. In the To column, add a number or select an object to send from the address book. ?

3. After selecting the payee, the payee will prompt you which card you are using now, and click OK. ?

4. A pop-up box will appear. Just switch the primary card/secondary card number.