Want to terminate post-paid account but account doesn't exist?

I am terminating a post-paid account on another's behalf, but I am unable to because this account does not seem to exist. 

 

Called 1633 from my own phone and when directed to input the post-paid account's number or their NRIC, I am informed the details are invalid. I don't know how I can reach an operator.

 

The account owner has also visited Starhub outlets and asked about this number, but the staff said that it "does not exist" and are unable to do anything about that number... I am not sure of other details in that visit.

 

Recently on 4th September we received a bill from this account even though the number has not been used for almost a year.

 

The situation around this very strange and even how this contract was made in the first place is foggy to me, so I am not sure how to continue on with this. Can I have some advice on what I should do next? Who should I contact that can help look into who is using the account and why there are no results on this number?

 

Thanks in advance.