WhatsApp can still work on the same account if you change the mobile number or SIM card. You can even move your WhatsApp account to a different smartphone.
You might think that a WhatsApp account is tied to your phone number, but that’s not true at all. WhatsApp (like other messaging services) sends an SMS text to your number as part of the setup and verification process so it’s understandable for people to think so.
Once WhatsApp is setup on a device, it’ll stay there until you uninstall it. Switching SIM cards makes no difference to WhatsApp. It will continue to work normally. We’ve done this ourselves many times as we travel.
Changing phone number – temporary
The WhatsApp account is linked to the phone number used at setup (the one that gets the initial text message). You can change to another phone number on the same device (e.g. change SIM cards on GSM phones) on a temporary basis.
Other people join you on WhatsApp using the phone number linked to your WhatsApp account. Give new contacts your original WhatsApp linked phone number so they connect with you on the service.
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WhatsApp across many countries
This portability is great for travelers who are temporarily away from home and their usual phone number.
It’s common to switch SIM cards while on the road because the cheapest/best option is almost always to use a local SIM in each country. But the WhatsApp connection remains unchanged no matter how many times you change SIM cards.
We know people who have gone through six or more SIM’s using the same WhatsApp account. The WhatsApp contacts don’t even know the person is travelling because the connection and linked phone number remain the same.
WhatsApp without a mobile/wireless phone number
It’s even possible to use WhatsApp without a mobile phone connection. Great when you’re overseas and don’t have mobile coverage (too expensive, too difficult).
An existing WhatsApp account already on a device will continue to work with only Wifi connections to exchange messages.
The Whatapp account must already be setup on a device when connected to a mobile network (i.e. the usual setup). Once that’s done only Wifi is needed.
Changing phone number – permanent
Most parts of the world have ‘number portability’ so you can change providers while keeping your original phone number.
If you have a new permanent mobile number it’s possible to migrate your existing WhatsApp details to the new number.
See https://faq.whatsapp.com/en/android/27585377 for the step-by-step process.
WhatsApp’s description makes it seem like you have to do this procedure each time you change SIMs but it only applies to permanent change of number.
Changing devices or smartphones
WhatsApp also has a method of changing device/smartphone while keeping the same number.
Their method isn’t clearly worded. It’s headed “Using your old account on a new phone” but then refers to ‘old number’ and ‘new number’.
Essentially you install and setup WhatsApp on the new device and verify it via SMS. The fact that it’s the same phone number as another WhatsApp account doesn’t matter.
Once WhatsApp is verified on the new smartphone, you can migrate your chats etc using WhatsApp’s steps.