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How to call singapore 
Know the timezone. 
Keep in mind the time in singapore when making a call. The time zone followed in singapore is utc/gmt + 8, or 8 hours ahead of greenwich mean time.
This means that it is 16 hours ahead of pacific standard time (pst) so if you were calling at noon from california, it would be 4 am in singapore. You can download an app or check online clocks for the current time in singapore.