Also happened to me when i ported out of Maxis to another telco. The staff mentioned that i need to settle an interim bill to port out the number, but I will receive a closing account final bill for the following month.
The collection team only recently started pestering me for the final bill to close the account since their audit have been questioning them about these unsettled final bills.

Eventually i settle the final bill since i don't them to keep spamming me on whatsapp