The confusion is understandable. A SIM card used to be the thing that carried your number, so installing a new one felt like changing your identity. A data only eSIM carries no number at all, which makes it less disruptive than a card, not more.
Once you separate the account from the card, the whole thing becomes obvious.
WhatsApp is tied to a number, not a SIM
WhatsApp registers an account to a phone number. That number is verified once, by a code sent in a text message, and after that the account lives in the app and in WhatsApp’s own systems. It does not read the SIM card again, and it does not care which SIM is providing the data your phone is using.
So a travel eSIM changes nothing. Your chats, your contacts, your groups and your profile all stay exactly where they were. The app simply uses whichever line has mobile data, and on a trip that should be the travel plan.
What a data only eSIM gives you
Most travel eSIMs are data only, ours included. No phone number, no voice line, no text messages. What you get is an internet connection in the country you are visiting, at a fixed price, with no bill afterwards.
That is enough for WhatsApp. It is a data application. Messages, photos, voice notes and voice calls all travel over the data connection, which is why a plan with no number is perfectly capable of carrying a conversation with someone on the other side of the world.
It is worth being clear about the one thing you lose, because it is easy to discover at the wrong moment. With no number of your own in the country, nobody there can ring you on a local line, and a restaurant, a taxi firm or a hotel cannot reach you on a number you gave them. In practice almost nobody wants this, since you are the one making the calls, and the number you hand over is the one in your WhatsApp profile.
How much data WhatsApp actually uses
Less than people expect, which is the reason a small plan usually turns out to be enough.
Text messages are trivial, a few kilobytes each, and a thousand of them would not fill a photograph. Voice notes sit in the middle. A photograph costs a few megabytes, a video clip considerably more, and a voice call over WhatsApp uses a small amount per minute compared with a video call.
The one habit worth having is on the sending side. Sending photos at full resolution and receiving every video in a group chat will use far more than reading and replying, and the phone will do all of it without asking. If you have a modest plan, let the photos upload over hotel wifi and keep the mobile data for the things you need in the moment.
The three arrangements that work
The first, and the one most people want, is a single WhatsApp account on your own number with the travel eSIM carrying the data. Nothing is registered, nothing changes, and when the trip ends you delete the eSIM and go back to normal.
The second is a local SIM in the second slot, bought for calls and data, with WhatsApp left on your own number. This is useful when you need to make ordinary phone calls locally, and it keeps your contacts messaging the number they already have.
The third is two WhatsApp accounts, one on each line. WhatsApp and WhatsApp Business are separate applications, and each can be registered to its own number. That is the arrangement for someone who wants a work account and a personal one, or a local number that outlives the trip.
The one thing that catches people out
Verification codes.
When you switch your own line off to avoid roaming charges, you also switch off your ability to receive text messages. Most things do not care. Two things do: any service that sends you a code, and any attempt to log in somewhere new.
Banking is the common case. A card payment, a new login or a security check will send a code by text to your own number, and if that line is off the code never arrives. The fix is simple and costs nothing to plan: keep your own line switched on so it can receive messages, and leave it out of the data business entirely. Calls and texts arrive, data does not, and the roaming bill stays at zero.
A note on calls over data
WhatsApp voice calls use a small amount of data per minute, far less than a video call, and they are the reason a data only plan is not much of a limitation on a trip. If you plan to call home regularly rather than send messages, allow for it when you choose a plan size, and how much data you need works through the numbers.
One more thing worth knowing if the number itself matters. If your own line is switched off for the whole trip, WhatsApp will still send and receive messages, because it is using the travel eSIM for data. The only thing you lose is SMS, and the only people who notice are the ones sending you codes.