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The iconic red telephone kiosk was the result of a competition in 1924 to design a telephone box suitable for London Metropolitan Boroughs. A design by Giles Gilbert Scott, a British Architect, was chosen. The box, to be known as the K2 was deployed in London in 1926. The post office suggested it be painted red.

The K6 was introduced in 1935, designed to commemorate the silver jubilee of King George V. It was a smaller version of the K2, and went on to be installed prolifically around the country. It is the most recognised and iconic telephone box, that many people around the world are familiar with.

Other versions of the red telephone box were designed and implemented but none were ever to survive the popularity of the K6.

Over 240,000 red telephone boxes were built between the 1920s and 1980s.
BT had replaced many red telephone boxes during the 1980s and 90s, leading to English Heritage to designate over 2000 as listed structures.

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites.

There are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures using A, B and C designations.

…read more at wikipedia Link