Why can't I get a text message on another card of Apple's dual-card mobile phone?

System: iOS 15. 1

Apple dual card another card can't receive SMS, which may be caused by the phone not enabling this number. Taking the mobile phone iPhone 12 as an example, the steps to activate this number on the Apple mobile phone are divided into three steps. The specific operation is as follows: 1 Click cellular network.

2 Choose a mobile phone number

Open this number.