URC destination interface 有4种
1. "mdm" Use modem interface to output URCs
2. "app" Use application interface to output URCs
3.“usb0-mdm“ 这里就不介绍了
4.“usb0-app“ 这里就不介绍了
通过 AT^SCFG? 命令读取出现在处于 "mdm"接口
这里还有一段介绍
If the AT^SUSB startup mode "None" is active, then the powerup value is "mdm", otherwise "app".
If the multiplexer is started with AT+CMUX and the AT^SUSB startup mode "None" is active, then automatically
the value "app" will be selected.
Parameter is global for all interfaces, volatile and will not be reset by AT&F.
If the modem interface is used, keep in mind, that if the interface is blocked by an active data connection, then this connection should be periodically suspended to check for pending URCs!