the the spirit of christmas compels me to not berate you for being so lazy that you cannot search the forum, or the internet.
there are only 3 cell phone service providers in singapore. and i recommend either
go to those web site, click on MOBILE, then MOBILE PHONE PLANS (two types, pre or post paid) and VALUE ADDED SERVICES to find the local minutes and IDD call packages they have.
all 3 will have similar plans that vary a little here and there. if you want cheap international calls, you're probably better off using skype or some other service like skype.
coverage for the most part, are also similar but in odd places around the island it can get spotty for one provider but not another.