With Dual SIM, you can add a second line to have two cellular plans on a single iPhone.
Setup a secondary plan from your mobile carrier or transfer another line if you already have one
When cellular setup is complete for your new plan, you might be prompted to set up Dual SIM right away.
Tap Continue to start.
You can have multiple lines, but you can only use two at a time.
You can label each one to identify which is which.
And tap Continue when you finished labeling your lines.
Next, select the line you want to use as your default line.
This will be the number your iPhone will use by default to make calls and send text messages.
Then tap Continue and choose which lines you want to use with iMessage and FaceTime.
You can select just one or both.
and choose which line will use cellular data.
The other line will be voice only.
If you'd like to be able to use the internet while you're making calls with your voice-only line,
you can turn on "Allow Cellular Data Switching."
When you're finished, tap Done.
If you want to make changes to a line or turn it off,
you can tap it for more options.
Your signal strength will be displayed
How to use Dual SIM
in the upper-right corner of your screen.
The top bars represent your cellular data line
and the bottom represents your other line.
When you're ready to make a call, open the Phone app and select a contact.
If you don't assign a line to the contact,
the last line you used to communicate with them
will be the default for calls and messages.
To assign a line, tap the option next to "default" under their name.
Then select a line to always use and tap Done.
If you dial a phone number that isn't in your contacts,
your iPhone will use your default line for the call.
You can change the line you want to use by tapping
the gray cellular plan label at the top of your screen
above the phone number you're dialing.
Just tap the line, and you're ready to make your call.
And that's how you use two lines on one iPhone with Dual SIM.