These GPO telephones were the very first "Bakelite phones" to be used in the UK. They superseded the previous Candlestick telephones (known as GPO Tel No 150), the candlesticks consisted of a metal bodied telephone instrument and a wooden "Bell Set" that contained both the bells and the telephone internals (known as the speech circuit).
Although the 200 series GPO telephones were Bakelite they were still provided in two separate parts, the "Telephone Instrument" and the Bakelite Bell unit. It was not until the introduction of 300 series telephones in 1937 that the idea of putting the bells inside the rotary telephone was developed.
The most common configuration of these 200 series GPO telephones in a house was as two separate items, with the "Bell Box" fastened to the wall connected to the telephone instrument by a braided cord (see picture above).
However the rotary phone could also be provided as a combined unit with the Bakelite bell box sandwiched between the top of the telephone instrument (the pyramid shape) and the base of the phone that contained the drawer. In this configuration the telephone was known as a "Combined Set". In recent years this configuration of the telephone has become popular with vintage telephone collectors as it takes up a smaller amount of desk space.
The first 200 series Bakelite telephones had the GPO designation of Tel No 162 and were first introduced into circulation in 1929. The next generation was introduced in 1934 (virtually identical in appearance) and known as GPO Tel No 232.
The 162 Bakelite GPO telephone instruments were paired with a Bakelite bell set No 25 or a wooden bell set No 1. Similarly the 232 telephones were paired with a Bakelite bell box No 26 or occasionally a wooden bell box No 1A.
Because these original GPO telephone bells are hard to find some antique dealers pair these old telephones with more modern bells such as the metal bell 64D. More modern bells such as the 64D are far easier to find and are a much cheaper option. However the GPO never paired 64D bells with these 200 series Bakelite phones when they were in service.
Although the 300 series telephones were introduced in 1937 both the existing 200 series telephones and the newer 300 series GPO telephones were maintained in parallel until 1959 when they were both superseded by the "Modern" Plastic 706 telephones.
Trivia: You will often see 200 series GPO telephones configured as Combined Sets referred to as "King Pyramids", however this term is actually a relatively new nickname believed to originate from antique dealers, it was never used by the GPO or it's customers.