Country Code vs. Area Code
Two levels of routing
A country code routes a call to a national numbering plan, and an area code routes it to a city or region inside that plan. In +44 20 7946 0958, 44 selects the United Kingdom and 20 selects London.
The split is different for every country. Some use fixed-length area codes, some use variable lengths, and some assign mobile numbers to their own code ranges rather than geographic areas.
When codes look similar
Do not confuse a country code with an area code that begins with the same digits. For example, +1 201 is the New Jersey area code 201 inside the North American +1 plan, not country code 20 (Egypt).
A complete international number always has the country code first. If a number starts with 00 or +, the following group identifies the country; the group after that is the area or mobile code.
Checking a full number
To verify a number, split it into country code, area code, and local number using the country's detail page here. Search the country first, then compare the leading digits of the number with the area code conventions shown by your carrier or the recipient.
Shared-country-code regions require an extra check: +1 covers many destinations, so confirm the area code before dialing rather than assuming the destination.