What dialing code is used in New York on an English mobile to call an English mobile in New York?

Replace the leading 0 of the mobile number you are trying to reach with the UK's country code +44. For example, 07700 900123 becomes +44 7700 900123, including the plus symbol.

(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.)