Register a new account over that phone number.
They can’t get into any previous accounts register with that phone number.
They could potentially manage to find the pin if the previous user really used a guessable one, but then again, they won’t be able to check the previous messages and the linked owner of that account will be warned of that new connection.
They can’t “take over” your account, but they can “override” it and delete yours.
How can they override it?
Register a new account over that phone number. They can’t get into any previous accounts register with that phone number. They could potentially manage to find the pin if the previous user really used a guessable one, but then again, they won’t be able to check the previous messages and the linked owner of that account will be warned of that new connection.
I don’t think that’s possible with a registration lock unless you are inactive for longer than 7 days.