How does Apple 12 switch numbers to send short messages?

Apple 12 switch numbers to send short messages, as shown below:

1, click information to enter, as shown in the figure below:

2. Enter the information interface and click New, as shown in the figure below:

3. Enter the new interface and click the recipient, as shown in the figure below:

4. Enter the recipient number or add an address book, as shown in the figure below:

5. After adding the recipient, the default mobile phone card name will appear. Click on the mobile phone card, as shown in the figure below:

6. The option of two cards appears, and switch to the card to be used, as shown in the figure below:

7. If the handover is successful, enter a short message and click Send, as shown in the figure below: