Sorry to hear that. This is not something new. If I'm not mistaken, the caller who called u is not from Celcom, but a debt collection agent. Their task is to call all defaulted acc user and get them to pay.
You might need to check with Celcom or their Collection department. If they have write-off ur acc, the outlet guys may not be able to check. But no harm trying to visit the Blue Cube or contact them as per below;
"As this is an account specific issue, please contact our direct support agent as we will need to have your personal account related information. You may click on this link https://www.messenger.com/t/celcom for assistance. Alternatively you may also call 1111 and speak to one of our Customer Service Consultants."
All the best!
Yeah... Celcom has been having this issue for a while. Usually when u port out, the final bill actually haven't come out yet. So, port out also no problem cuz no outstanding at that time. But actually got one final bill & they always don't (or 'forgot') to tell abt this.
I would call/contact Celcom just to clarify which bill is this for just to be sure.
Hopes this helps (*•̀ᴗ•́*)و ̑̑