Int'l trip to US — when to activate eSIM?

Thanks for the info.  I'm still using my VPN to write this post.  The problem is that the entire domain blocked my primary IP.  So even if I try to access T-Mobile support pages (which have nothing to do with this forum), my IP is blocked.  But for now, the VPN is a work around.

I contacted T-Mobile via Twitter.  They said they couldn't access this forum thread (for reasons I don't understand), so I sent them my opening post and select replies from yourself and syaoran .  They replied back to say activation can be done while I am still in Japan by way of using WI-FI alone.

Below is a copy/paste of what the T-Mobile support person on Twitter told me...

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Here is the guidance for activating your eSIM on an iPhone 15 Pro Max while in Japan before your travel to California:

Pre-Activation Steps:
• Ensure your iPhone is connected to Wi-Fi. 
• Update your device software to the latest version. • Remove the physical SIM card if you will not be using dual SIM.

Activation Process:
• Go to Settings > Cellular > Add eSIM/Set Up Cellular on your iPhone.
• Follow the prompts to download and activate the eSIM profile.

Timing and Cellular Settings:
It’s recommended to activate the eSIM just before your trip. You can do this the morning of your departure while still connected to Wi-Fi in Japan.

To prevent roaming charges:
• Activate the eSIM while Wi-Fi is connected and ensure cellular data is turned off.
• Go to Settings > Cellular and disable Cellular Data or ensure the eSIM line is not set as the primary data line.
• Once your eSIM is activated, you can label it accurately for future ease of use, e.g., "Travel".

Important Considerations:
• Enabling Wi-Fi is crucial during the eSIM download process. 
• Make sure not to actively use the eSIM for data or calls until you arrive in the United States to avoid potential roaming charges.

Troubleshooting:
• If activation issues arise, ensure the device is still connected to Wi-Fi and try re-downloading the eSIM profile from the settings.
• Should any errors persist, contact T-Mobile support for further assistance.

For further details, you may refer to: Your device settings and activation tutorials available on T-Mobile’s support page.