What area has STD code 0203?

You need to specify the country, since several different countries refer to telephone area codes as "STD Codes." See the related questions below.

There is no area code 0203 in the United Kingdom. However, there is the area code 020 (or +44 20), which is for London. So, an 11-digit UK number beginning 0203... is just a regular eight-digit London phone number prefixed by the area code of 020. It should be written: (020)

In India, area code 020 (or +91 20) is Pune, Maharashtra. Again, the '3' would be the first digit of the local number.

An international telephone number beginning with +20 3 (dialed as 00 20 3 from many places) is in Alexandria, Egypt.

(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.)