Summary

I purchased and am currently using the SKT Korea eSIM (15-day plan).
While keeping my U.S. number (primary line), I added and installed a Korean eSIM.
However, I experienced an issue where iMessage using my U.S. number (+1 209-XXX-XXXX) stopped working.
Even with “Allow Cellular Data Switching” turned on, messages were not being sent or received properly.

Causes

Depending on your phone and message settings, there are typically three main causes:
1. Confusion in Number Settings
After installing the Korean eSIM, the iPhone may mistakenly switch the default iMessage number from your U.S. number (primary line) to the Korean number (secondary line).

2. Temporary Deactivation of Authentication
During the activation of the Korean eSIM, the U.S. line may briefly lose network connection.
This can cause iMessage to disconnect from Apple’s server, resulting in a loss of authentication.

3. Sending/Receiving Line Disabled
In some cases, the U.S. number may become unchecked (disabled) in the “Send & Receive” settings within iMessage.

Solutions

Depending on your phone and message settings, there are typically three main causes:
Follow the steps below in order—this resolves most cases:

1. Check Number Settings
Go to: Settings > Messages > Send & Receive
Make sure your U.S. number is checked (✔)

2. Restart iMessage
Go to: Settings > Messages
Turn iMessage OFF → wait 10 seconds → turn it back ON
→ This will re-authenticate with Apple’s server

3. Turn OFF “Allow Cellular Data Switching”
Go to: Settings > Cellular
Set cellular data to the Korean eSIM
Turn “Allow Cellular Data Switching” OFF to avoid unexpected charges

4. Keep Both Lines ON
Go to: Settings > Cellular
Keep both:
Primary line (U.S.) → ON
Secondary line (Korea) → ON
⚠️ If you turn off the U.S. line, iMessage authentication will also be disconnected.

Conclusion

Even if your primary line (U.S.) is turned ON, as long as “Allow Cellular Data Switching” is OFF,
all data will be used only through the Korean eSIM.
You can continue using iMessage with your U.S. number without any roaming charges—so use it with confidence!

Support

If you purchased and are using a Korean eSIM through  NeoKOSIM website,
feel free to contact us anytime if you have any questions.