I'm constantly calling all over the world dealing with a variety of national formats for various countries. However, there are more variations than I can remember - so, when in doubt, check out the official source from the ITU below.
DIALING PROCEDURES (INTERNATIONAL PREFIX, NATIONAL (TRUNK) PREFIX AND NATIONAL (SIGNIFICANT) NUMBER) (IN ACCORDANCE WITH ITU-T RECOMMENDATION E.164 (02/2005))(March 1st, 2010 Position)
In case you are not familiar with E.164, the document above defines the "recommended" phone number format for every country in the world.
This includes (with examples for the UK):
- Country Code (44)
- International Prefix (00)
- National Prefix (0)
- How many digits in National Numbers (7 to 10 digits)