The first  line of your post already gave it up that it's  a scam attempt, no bank will call you and start asking you to submit a OTP sent to you by claiming to help you .
And to answer your question, you might have submitted your account number details somewhere, could be Social medias give away threads (part of their tricks) or a worker of theirs just as you've mentioned (probably customer care or something..) there are lots of ways . They probably will try to access it through banking app by requesting Forgot password.. it's a well known scam.