This is the RTT 56 B, the well-known Belgian kettle phone. The B stands for BTMC, Bell Telephone Manufacturing Company. There was also an RTT 56 A version, where A stands for ATEA. The number 56 stands for 1956, the year in which this model was introduced.
The RTT 56 B differs only a little bit from the RTT 56 A. Only the model dial used and the letter B on the underside betrays that the phone was made by BTMC.
For more information on the RTT 56, read the article on the RTT 56 A.
Here is an article on restoring an RTT 56.