I called singtel to inquire. The cust. svc. officer informed that Singtel is 'aware' of this issue and has 'escalated' this to Apple.
Simply put, he's saying that its Apple's fault as he mentioned this 'should-be-fixed' by Apple on their next update.
In my case, my nano SIM was from my iPhone 5 and ever since upgrading to the iPhone 6 Plus, I'm in the boat where the activation doesn't seem to be working for my mobile number.
Since @crazychaz SIM change worked, i'm wondering if yours is from your iPhone 5? or 5S? Also, only official Hello Shops can do this change and not from any exclusive retailer?
My SIM was from an iPhone 5.