Chongqing was part of Sichuan and it had the 811 city code until
8 March 1997. In April 1997, Chongqing became the fourth
municipality under the direct jurisdiction of the central
government.
All counties in Chongqing with old 811 code now has the new 23 code.
Chongqin local phone number was changed to 8 digits on March 8, 1997.
The rules are xxx-xxxx to 6xxx-xxxx for regular numbers, and
xxx-xxxx to x8xx-xxxx for mobile phone numbers.