The "green logo" which Swiss Tony has described is Starhub. Starhub is the only telco that charges calls by per second, as opposed to per 6sec/minute (can't remember which) by the other two telcos.

Here are the links the three major telcos in Singapore and their prepaid services

Singtel

M1

Starhub

Make sure you bring your passports along with you when purchasing your prepaid cards.