Tel scheme overview
This scheme is a tel URI in a format as specified in RFC 3966 and is followed by a phone number optionally formatted according to E.164 formatting specifications. E.164 is the international standard for how global phone numbers are allocated and used. The tel scheme deals with two kinds of phone numbers:
Global phone numbers
E.164 numbers like tel:+1 317 872 3000 (a U.S. number) and tel:+31 75 657 3394 (a Netherlands number). Calls to these numbers should reach the same recipient regardless of the location of the person placing the call. These are usually written with a leading + (plus sign) to indicate that they are global numbers.
Local phone numbers
Number like tel:411 (U.S. local directory assistance) or tel:31756573394. If one dialed the former from Minneapolis, they could request directory assistance for the Minneapolis area. If dialed from Atlanta, it would reach directory assistance for the Atlanta area. Similarly, if one dialed the latter number from New York, it would be interpreted as a call to U.S. area code 317. (U.S. numbers discard superfluous trailing digits.) However, it is the same sequence of digits listed as “a Netherlands number” above.