The plus (+) sign and international calls
You've probably noticed that many international phone numbers begin with a plus sign. Here's an example: +44 700 000 0000. Some of you may know that 44 is UK's country code, but what does the plus sign in front of the country code mean?
The plus sign vs international prefix (a.k.a. IDD or exit code)
The sequence required to dial a foreign number is: International prefix - Country code - National subscriber number. The plus sign before the country code means "add the international prefix" of the country you are dialing from. So, using the example above, if you were dialing from the US you would replace the plus sign with 011, while, if dialing from one of the numerous countries using international prefix 00, you would instead have 00 44 700 000 0000.
Here's a list of international prefixes.
Can I actually dial the plus sign?
With the advancement in technology and growing popularity of cellular phones you can now simply enter the plus sign before the country code. Cell phone devices allow it and wireless networks automatically acknowledge the international call. In this case you no longer have to enter the international prefix. The big advantage with using the plus sign is that you don't have to add a different international prefix for each country you are dialing from.
Long pressing 0 (zero) will dial + on most wireless devices.
A recap
If you see an international number that has a plus sign before the country code you either dial + or replace it with your country's international prefix.