+ is the character for the international calling code. You need to have this prior to the phone number. It doesn't hurt to have them in all the time.
You then need the country code. 1 is the country code for the US. 44 is the country code for the UK. 49 is the code for Germany. Here is a link to a list of country codes. Check out the by zone section, I find it easier than the "at a glance" list. List of country calling codes - Wikipedia, the free encyclopedia
Sometimes to call a phone number from another country you have to remove a leading zero from a phone number. If the number starts with zero and it doesn't have the +, etc. in front of it try removing the zero. Sometimes people put the zero to remove in paranthesis like this: (0).
A US number would be: +1 800 555-1212
A UK number would be: +44 1122 345 678