I used to have a SingTel prepaid that can be purchased at any 7/11 or convenience store in Singapore. It will work in both S/M. Unfortunately I got a permanent post paid card so not up on prepaids for these two countries.

Check prepaidgsm.net - I believe SingTel, StarHub and one other carrier in Singapore. Malaysia might be more restrictive, not sure.

Also 018 is a good access for international calls when using the StarHub prepaid.