Celcom FiRST is not prepaid lorrr.

What you paid when signing up all is used to offset first bill. It's "upfront payment", not "deposit".

From 2nd bill onward it will really be use first, bill later.

When you terminate or MNP, what and how much you used between previous bill till termination is not yet determined, regardless how you wish that such timeliness is the case.

As someone who has "been there, done that" in telco backoffice, I can confirm that while charging for postpaid is done real time, the unified accounting of the various chargeable elements for postpaid does not happen in real-time.
Telco charging is a complex and expensive operation on its own and a simple error can escalate into major monetary discrepancies and "shouting" matches between operators, partners, and customers.
As much as we'd like you to pay your dues immediately when you decide to terminate our relationship, this is not all-you-can-eat for one price, and we can't determine exactly how much you have used at your whims, fancy, and timing.

Just like electricity and water, you are free to eat as much as you can; but wait for you next bill for us to account properly how much you have actually used.

I personally have seen correct items that should have gone appeared in previous bills, appearing in next bill. That is how delayed the accounting can be.

(We can make it real-time, but will ALL of us be receptive to paying the same expensive charges as what prepaid users are now "enjoying" the the real-timeliness? it can't be some want and some other decline. either we're all all in the same boat or none at all.)
Yang merajuk,

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Customer Service
Telco XYZ