Weird

from the other thread last week (all SIN specific info here)
1) +800 2747 5500 was working from my M1 blackberry - busy today
2) 021 800 2747 500 is working from personal M1 cell today (021 being M1's discounted IDD service that I use- the connections seems to be uber feeble)
3) Been unable to figure out how to get this to work from my office (9 001 being our standard idd dialing, 9 001 800 xxx gives a busy-like tone, 9 800 xxx is invalid, while other combinations give a fast beep-beep)

and, from my little experiment, i think that there are a very small number of 'free' lines that work in parallel this way. #2 above got me a busy tone the second time around.

But then, why bother with any this? Singtel and M1 (and i'm guessing Starhub too) have cheap packages (like SGD5/month) that give you the IDD 019/021 (and i suppose 018 in starhub's case) calls at local call rates.
And most of their promotions throw these in for free (like my personal cell connection, taken at an employer specific promotion). I know that the places that I need to call (US,HK,India,SEA) are on the 021 list and i just prefix 021 to most numbers while saving them.

I have also used +800-2747-5500 from a prepaid cell in India (had to call MPC to sort something out, when i was at the check-in desk. Had almost no talktime left, and was quite grateful for the worldwide toll free access)