I find that Singtel has a strange way of billing, where it's not based on the first day you joined or start the contract/plan.
for my case, I joined them on 8 Jan but they billed me from 8 to 22 Jan, and charged me in advance from 23 Jan to 22 Feb. my 'billing period' is hence from 23rd of month X to 22nd of month (X+1)
I just signed up for the $15.28/mth plan recommended by
@xiaofan - hopefully it won't be rejected. I expect them to refund me the pre-payment made until 22 Feb... say if I'm successful in the new plan wef 9 Feb then they should be charging me the new plan from 9 to 22 Feb and refund me the amount I have paid on the old plan from 9 to 22 Feb.
... does it make sense?