Losing a phone is a security problem before it is an inconvenience. The WhatsApp account on it can be misused, and private conversations can end up in someone else’s hands. The account can be recovered — but the route runs through your mobile operator, not through WhatsApp.
What to do, in order
First, get back control of the number. Contact your operator as quickly as possible and have the old SIM blocked. Then obtain a replacement SIM for the same number and activate it.
Put that SIM in another handset, open WhatsApp, and register the account again. Registration needs the verification code sent to the number, which is why control of the number is the entire problem: without it, there is no ordinary route back in. Once registration completes on the new phone, the account is automatically logged out on the lost one.
What WhatsApp cannot do for you
It cannot deactivate the account on your behalf, because it has no way to verify that the number genuinely belongs to the person asking. It cannot locate the missing handset either. And you cannot log the lost phone out from WhatsApp Web or the desktop app.
That combination is worth sitting with: for the hours between losing the phone and activating a replacement SIM, the account stays live on a device you do not have.
Your chats are a separate question
Registering on a new phone does not guarantee old conversations return. Messages come back only from a backup made beforehand to a Google account. If there is no backup, the chats are gone — which is the argument for turning backup on now rather than reading this advice after the fact.
If you cannot get a replacement for the old SIM at all, try the same number first. Failing that, WhatsApp’s Change Number feature can move the account to a new number — but that requires access to the old account and an active new number, so it is a plan for a phone you still have.
The Bangladesh specifics
A replacement SIM is issued at an operator’s own service centre against biometric verification, with the national ID of the person the number is registered to. Two consequences follow. If the number is registered in someone else’s name — a relative, an employer, the shop that sold you the SIM — that person has to go in, and your recovery now depends on their availability. And the centre has opening hours, which is exactly the gap described above.
Second: the first hour is the dangerous one. A stolen phone with WhatsApp already unlocked is most often used to message the owner’s contacts asking for money, because those messages arrive from a trusted name. If you cannot reach an operator immediately, tell a few people in your closest groups what has happened. A warning costs nothing and defeats the whole scheme.
The setting that makes all of this unnecessary
Turn on two-step verification in WhatsApp — Settings, Account, Two-step verification — and set a six-digit PIN. From then on, registering your number on any handset requires that PIN as well as the SMS code. Somebody who takes your SIM, or persuades an operator to issue a replacement, still cannot take the account.
It takes about thirty seconds and it is the single highest-value thing in this article. Do it before you need it, and keep a note of the PIN somewhere that is not the phone.
Related: WhatsApp calls by link, and the question each link raises.
