Hello SinghIsFido,
Welcome to the community!
I understand it might be unsettling to have your number previously owned by someone else. However, can be entirely normal. The number of phone numbers is finite. From time to time, new overlays are created to expand the existing number allocation. However, with the number of users ever increasing, even those overlays fill are quickly exhausted. The practice of recycling numbers is commonplace and necessary. Generally, there is a minimum period where those cancelled phone numbers are not in circulation (see here).
While a mobile provider assigns your phone line with a phone number, they have no control over any existing accounts with other services associated with that number. With regards to WhatsApp, the previous owner of the phone number did not disassociate the number with their WhatsApp account. However, the previous owner does not have any access to the WhatsApp account you activate with your new phone number. Your conversations and other WhatsApp data are secure (see here). You might consider reaching out to WhatsApp and informing them that you are the current owner of that phone number.
Since your post is with regards to WhatsApp, I would be remiss if I did not mention the apparent bug with the app. While it's not mentioned on their website, it appears as though the app will automatically switch to using mobile airtime minutes (within the app) if it deems the internet connection is insufficient for voice calls. Unfortunately, it has happened to many people across Countries and even Continents (see here). One suggestion to prevent the switch would be to enable Airplane or Flight mode and manually enabling Wifi prior to making the call(s). Doing so should prevent the app from switching to mobile minutes if the Wifi internet connection is not sufficient. However, it also means that you wouldn't be able to make WhatsApp calls using a mobile data connection.
Hope this helps 😀
Cheers
