What is the telephone area code for a mobile in Singapore?

The country code for Singapore is +65. There are no city codes or area codes.

Mobile numbers begin with +65 8 or +65 9, not including +65 800.

Landline numbers begin with +65 6.

VoIP numbers begin with +65 3.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)