Shadowfax — No return pickup but asking otp

Hi,
I have not placed any return pickup till now but getting call from ShadowFax number [protected] which is pressurising me to give an otp which they are sending. The person says that please give otp for return pickup and when I say that there is no return pickup then he says that ok, if you want to cancel the return then please provide cancellation otp which we have jist sent you. And they keep sending me otps and asks for it.