There are some “non-standard” connections between regions of the world.
North American Numbering Plan (NANP)
The NANP is a numbering plan consisting of Canada, the United States, a majority of Caribbean regions, and U.S. territories in the Pacific.
It shares the country code +1, as well as a ten digit dialing plan and similar number formats.
French numbering plan regions
Many of the territories and overseas department of France can call between each other with a similar dialing sequence as placing a call from one side of France to another.
Republic of Ireland & Northern Ireland
Ireland and Northern Ireland share a special arrangement for calling between each other despite being in different country codes.
Callers from Ireland can reach Northern Ireland by replacing prefix 028 with prefix 048, as well as through the traditional call sequence to the United Kingdom of 00-44-phone number.
Russia and Kazakhstan
East African Posts and Telecommunication Corporation countries (Kenya, Uganda, and Tanzania)
Kenya can be called by dialing 005, Uganda by dialing 006, and Tanzania by dialing 007.
This three-digit code replaces the need to dial the exit code and country code in the call sequence.