Yes, I just done so.
As you may be already aware, the $15.28 plan is only for "existing" Singtel customers, not for "new" (i.e. port-in or new number) Singtel customers.
If you want the $15.28 plan ASAP as someone who is not a existing Singtel customer right now, you should go for the cheapest NO-CONTRACT plan (note, NOT the 1-year contract plans like Enhanced Lite, or else you are stuck on 1-year contract for 1 year duration). You have to pay early termination fees to leave anytime earlier before 1 year, as you would not have paid to Singtel the full 12-month worth of monthly contract fees.
The cheapest no-contract plan for new signups right now would be the Enhanced Core plan at $40 (if you have CIS, the Enhanced Core would be $4 cheaper at $36).
Then, after minimum 1 month on the Enhanced Core plan, pay for your 1st month usage after 1st bill generated (i.e. $40), then use the $15.28 form found
here and sign up for the $15.28 plan. You will be eligible this way for the $15.28 plan as an "existing" Singtel customer.
Tried and tested personally the above-mentioned process, only very recently the whole process from Aug to Oct this year. It works. Hope this helps.