I do that every month. Lazada also can, qoo10 also can. Whichever got better shopback at that time. Even better when Lazada's seller forgot to put 'no voucher accepted' and I use one of the 20% vouchers.

You need to make sure your Lazada registered phone number is the number you're reloading. Then you buy, once successful you will get an SMS saying your bill been paid with that amount.
We're talking about U mobile only. All other telco postpaid cannot be paid using prepaid credit.