This is the RTT 56 A. The A stands for ATEA. There was also an RTT 56 B version, where B stands for BTMC (Bell Telephone Manufacturing Company). The number 56 stands for 1956, the year in which this model was introduced.
The Belgian state telephone company, RTT (Regie der Telegraaf en Telefoon/Regie de Telegraphe et Telephone) asked her 2 suppliers, ATEA and BTMC, to come up with a standard telephone together. This design was based on a prewar design by ATEA, the type 50/51 ATEAPHONE. This ATEAPHONE had an all metal housing and bakelite handset. With some adaptations, such as the addition of the carrying handle, this became the RTT 56.
Both producers used their own model dial and there were a few other minor differences.
At the front there is a small shield with the letters RTT. Sometimes there is a lion on it, which is also a logo used by RTT. The lion is the older RTT logo. As it was not very well known with the general public, another logo was introduced that did show the lettering RTT clearly.
These telephones come with and without a small white earthing button. Besides the black version, there is also a white version. Some are painted white after they left the RTT inventory. These have the RTT logo in black, instead of the white as with the original white ones.
Life after RTT
After RTT phased them out, many were sold on to 3rd parties. Many have received some sort of make over. Usually this entails that the original black paint was removed and the body shell got a copper treatment.
Many have been resprayed in a different color. So you may find not just a white one, but also red, green and blue versions. These are not their original colours.
On the sides of the telephone you can often find an elaborate decal, with nice swirls and the text Bell Telephone Mfg Company. See my RTT 56 B. These decals are not original and applied after their the telephone left the RTT.
Versions that made for private installations have undergone the same fate. They were disassembled together with RTT examples and reassembled randomly. This leads to a lot of refurbisched mixed up telephones today.
An original RTT example is hard to find.
RTT chose not to destroy their surplus materiel. Because of this you can find the RTT 56 often, all over the world, in all sorts of variants.
Here is an article on restoring an RTT 56.