How does Apple send short messages with the secondary number?

1. Click the "Edit" icon in the upper right corner of the "SMS" page, as shown:

2. Then enter the other party's mobile phone number in the box above or click the+sign to select a contact in the address book, as shown in the figure below:

3. After entering "Mobile Phone Number", the current default "SIM Card" will be displayed, and then click.

4. Select "Secondary Number" for the next operation, as shown in the figure:

5. The "secondary number" displayed on the editing page has been set successfully, and you can use the "secondary number" to send short messages.