What an eSIM actually is.
An eSIM is a digital SIM profile, downloaded and activated directly on the phone rather than inserted as a small piece of plastic, and most phones sold in recent years support at least one alongside a traditional physical SIM. The practical result is a phone that can hold two active numbers at once, each with its own calls, texts and data plan, switchable from the settings menu rather than requiring a second physical device carried alongside the first.
This is not a privacy tool by itself so much as a genuinely useful piece of infrastructure that happens to make several privacy habits far easier to keep, chiefly by giving a second number somewhere to exist without a second phone in another pocket.
Setting one up.
Most carriers issue an eSIM as a QR code, scanned once through the phone's own settings menu, usually under Mobile Data or Cellular, then Add eSIM. Activation typically takes a few minutes, after which the phone shows two lines in the same settings screen, each nameable so it is obvious at a glance which is which. A handful of providers offer eSIM-only plans built specifically for exactly this second-line use case, priced modestly since they usually cover calls and texts without a large data allowance.
The phone's dual SIM settings let you choose which line handles data by default, which line rings for calls, and, usefully, which line each individual contact or messaging app should use, so a second number can sit quietly in the background until it is specifically needed.
What a second number is genuinely useful for.
Keeping a personal number off forms, sign-ups and one-off contacts that do not need to reach the number your close contacts actually use is the most common reason people add one. It also solves the shared or work phone problem cleanly, since a second number can run alongside a first without touching either the physical SIM slot or the phone's operating account, useful for anyone weighing the boundary between a company device and personal messaging.
For travel specifically, a local eSIM purchased for the length of a trip gives a working local number and data plan without swapping a physical card in and out of the phone at an airport counter, one of the more genuinely convenient uses of the technology rather than a privacy workaround.
Messaging apps and a second line.
WhatsApp, Telegram and most messaging apps tie an account to a specific number, so a second eSIM number can carry its own separate WhatsApp or Telegram account entirely, distinct from the one tied to your primary line. Some phones support running two instances of the same app at once, one per number, through a built in dual apps feature; where that is not available, the second account simply lives on a second phone or an old spare device kept for exactly this purpose.
Whichever route suits your phone, the result is the same: two separate accounts, two separate histories, and no need for the number that matters most to you personally to appear anywhere it does not need to.
Multiple profiles, and a practical limit.
Most phones can store several eSIM profiles at once even though only one or two can be active for calls and data at any given moment, which means a travel eSIM from a previous trip can sit dormant on the phone rather than needing to be deleted and reinstalled each time. Exactly how many lines can be active simultaneously, one physical plus one eSIM, or two eSIMs together, depends on the specific phone model and sometimes the region it was sold in, worth checking against the phone's own specifications before assuming a particular combination will work.
Removing a profile that is no longer needed is a clean, complete action rather than a partial one, deleting the eSIM from the settings menu the same way it was added, which also stops that line's carrier from billing for it going forward. Doing this promptly after a trip, rather than leaving old travel eSIMs to accumulate, keeps the settings menu tidy and avoids any confusion about which line is currently active.
A few honest limits.
A second number is not invisible by itself, since carrier billing still ties an eSIM to whoever pays for it, and it does nothing on its own for location privacy, covered in more detail elsewhere on this site. It is a genuinely useful piece of separation, not a disguise, and works best alongside the other habits already covered rather than as a single fix for everything at once. Treat it as one honest layer among several, useful precisely because it is simple, rather than as a substitute for the phone lock and app permission habits that matter regardless of how many lines a phone happens to carry.
How a second line fits a booking.
Planning a travel companion day through Schiphol or a longer trip beyond it is a natural moment to set up a travel eSIM anyway, and a companion filed under petite or black hair is booked through whichever number you happen to be using that week, since the thread itself is what matters, not the line it arrives on. The booking page covers the message either number needs to carry.