Samsung SM-J260M/DS User Manual page 19

Basic
Using dual SIM or USIM cards (dual SIM models)
If you insert two SIM or USIM cards, you can have two phone numbers or service
providers for a single device.
Changing the display name of SIM or USIM cards
Launch the Settings app, tap Connections → SIM card manager, and then select a
SIM or USIM card to set a display name for each card.
Setting preferred SIM or USIM cards
When two cards are activated, you can assign voice calls, messaging, and data services
to specific cards.
Launch the Settings app, tap Connections → SIM card manager, and then set the
feature preferences for your cards in PREFERRED SIM FOR.
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