Hi Lizzie,
The international call prefix in Europe is 00 while in the US it's 011 but the question is which dialing plan applies to your SIM card because today many operators have included a feature called "call completion" or "call correction" which allows subscribers using their domestic dialing plan abroad (e.g. offered by Starhome:
http://www.starhome.com/call-completion.html). So even if you were supposed to dial 01144 from the US to the UK, the network could still connect you if you dialed 0044. Some networks are even configured to add the country code if you have forgotten it and if the dialed number doesn't exist in the visited country.
In any case, as rfranzq wrote, the best practice is to store all your numbers in the international format where + replaces the regional international prefix and you will never need to care about dialing plans again, regardless where you go to.
Here are some examples how numbers need to be entered correctly:
example for US number: 555 1234567 becomes +1 555 1234567
example for UK number: 02 12345678 becomes +44 2 12345678
example for German number: 030 12345678 becomes +49 30 12345678