Where in the UK is area code 01303?

In the UK, area code 01303 (or international +44 1303) is Folkestone.

An international number beginning with +1 303 (dialed as 00 1 303 from many places) is Denver, Colorado, USA.

(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, and many other countries use different prefixes.)