Changing between primary and secondary lines for SMS - IPhone Xr

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True

Option to change between Primary and Secondary lines for messages is available only for NEW messages

If you start from an existing conversation, you cannot change the line

False

Even if you have had previous conversation with a contact you can change line

Start by changing from Primary to Secondary first or Vice Versa

• Use Primary as your default line: If you select this option, Primary will be used by default for voice, SMS, Data, iMessage, and FaceTime. Secondary will be available just for voice and SMS.
• Use Secondary as your default line: If you select this option, Secondary will be used for voice, SMS, Data, iMessage, and FaceTime. Primary will be available just for voice and SMS.
• Use Secondary for cellular data only: You might want to select this option if you're traveling internationally and you want to keep Primary for voice, SMS, iMessage, and FaceTime. This will allow you to use Secondary for data.

On models with Dual SIM, tap iMessage & FaceTime Line, then choose a line. You can register only one of your phone numbers for iMessage and FaceTime. Your other number can’t send or receive messages using iMessage, or make or receive FaceTime calls.

Set up iMessage and SMS/MMS on iPhone

If you can't send or receive messages on your iPhone, iPad, or iPod touch