Telephone numbers in China are organized according to the Chinese Telephone Code Plan. The numerical formats of landlines and mobile phones are different: landlines have area codes, whereas mobile phones do not. In major cities, landline numbers consist of a two-digit area code followed by an eight-digit inner number. In other places, landline numbers consist of a three-digit area code followed by a seven- or eight-digit internal number. The numbers of mobile phones consist of eleven digits. When one landline is used to dial another landline within the same area, it is not necessary to specify the area code. The target number must be prepended between different regions with the trunk prefix, which is 0. Calling a mobile phone from a landline requires the addition of the "0" in front of the mobile phone number if they are not in the same area. Mobile to landline calls requires the "0" and the area code if the landline is not within the same place. Mobile to mobile calls does not require the "0" outside mainland China. The Special Administrative Regions of Hong Kong and Macau are not part of this numbering plan and use the country codes +852 and +853, respectively. Mobile phones In mainland China, mobile phone numbers have 11 digits in the format 1xx-XXXX-XXXX (except for 140–144, which are 13-digit IoT numbers), in which the first three digits (13x to 19x) designate the mobile phone service provider. Before GSM, mobile phones had 6-digit (later upgraded to 7-digit) numbers starting with nine. They had the same numbering format as fixed-line telephones. Those numbers were eventually translated into 1390xx9xxx, where xx were local identifiers. The oldest China Mobile GSM numbers were ten digits long and started with 139 in 1994, the second oldest 138 in 1997, and 137, 136, 135 in 1999. The oldest China Unicom numbers started with 130 in 1995, the second oldest at 131 in 1998. Keeping the same number over time is somewhat associated with the stability and reliability of the owner. The 5th to the seventh digit sometimes relates to age and location. China's mobile phone numbers upgraded from 10-digit to 11-digit, with 0 added after 13x, and thus HLR code became 4-digit long to expand the capacity of the seriously fully crowded numbering plan. In 2006, 15x numbers were introduced.In late 2008, 18x and 14x (for data plans or IoT) were introduced. In late 2013,