An eSIM is a digital SIM card built into the phone. Instead of putting a plastic card in, you download your plan. Many phones can hold several eSIMs and use two numbers at the same time. iPhones sold in the US from the iPhone 14 onward have no slot for a plastic SIM at all.
Your phone must be unlocked and support eSIM. On iPhone: Settings, General, About, look for "Digital SIM" or "Available SIM". On Android: Settings, Network and internet (or Connections), SIMs.
Connect to Wi-Fi. The download needs internet.
Have the carrier's QR code, or install the carrier's app.
Open Settings, then Cellular (or Mobile Data).
Tap Add eSIM (on some versions, Add Cellular Plan).
Choose Use QR Code and point the camera at the code. If the carrier sent a link or uses an app, follow that instead.
When asked, give the new line a name such as "US" and keep the old one as "Home".
Choose US as the default line for cellular data. Turn off "Allow Cellular Data Switching" so your home line does not use expensive roaming data.
Choose which line to use for iMessage and FaceTime.
On Android (Samsung, Pixel and others)
Open Settings, then Network and internet (Pixel) or Connections, SIM manager (Samsung).
Tap SIMs, then Add SIM or Add eSIM.
Choose Download a SIM instead? or Scan QR code, and scan the carrier's code.
Name the lines "US" and "Home".
Set the US line as default for mobile data, calls and texts.
Turn roaming off for your home line.
Keep the home number alive
You can keep your home SIM in the phone and active for bank codes, while data and calls go through the US line. Texts to your home number usually still arrive while roaming, but some home carriers charge for each text received abroad. Ask them before you travel.
Deleting an eSIM by accident. A deleted eSIM cannot be put back by you. The carrier must send a new one, which may take a call or store visit. Never tap "Delete eSIM" unless you are sure.
Scanning the QR code twice. Most codes work only once.
Leaving data roaming on for the home line, then getting a large bill.
Changing phones: ask the carrier how to move the eSIM before you reset the old phone. On many phones there is a "transfer eSIM" option.