Originally Posted by
Mellified
According to the documentation the iPhone is single carrier call at a time. Data will switch to the other line though.
https://support.apple.com/en-us/HT209044
You can have both SIMs active for voice calls. You designate one as the default for outgoing calls but you can designate either one to use for a specific call. It will also remember that choice for later, but you can always override it any time.
If you're on a call and another call comes in on the other line, it goes to VM. Is that what you are referring to?
-David