Bear in mind that with SingTel at least, the pre-paid credit cannot be topped-up by yourself when you are not in Singapore unless you have a Singapore bank account (and internet access).
It is possible to get someone else who is in Singapore to do this whilst you're in Malaysia but they'll need a Singtel account too.

Also don't forget that you have to pay for received calls as well as outbound calls both in Singapore and Malayisa, so you can find your balance dropping sooner than you expect.

I have no experience with the other carriers.