How does Apple 13 set up an uninterrupted telephone network?

At present, the dual-card dual-standby dual-pass function of many mobile phones is realized by canceling the high-definition network call function. After this function is enabled, you can make phone calls continuously. Then how did Apple 13 set up its mobile phone continuously?

How does Apple 13 build an uninterrupted call network? Apple 13 needs to turn on the VoLTE function when it is set to call the Internet. This function needs to be activated by calling the operator to which the SIM card belongs or sending a short message. Take Unicom VoLTE as an example,

1. Click the SMS icon on the mobile phone desktop to enter the editing information interface.

2. Send DGVOLTE and reply Y to 100 10 to start the VOLTE function.

This paper takes iphone 13 as an example and applies it to iOS 15 system.