The Nippon Telegraph and Telephone Corporation , commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. It was founded on August 1, 1952.

Ranked 55th in Fortune Global 500, NTT is the fourth largest telecommunications company in the world in terms of revenue, as well as the fifth largest publicly traded company in Japan after Toyota, Mitsubishi Corporation, Honda and Japan Post Holdings, as of September 2019.

The first commercial 1G mobile network in the world was launched by Nippon Telephone and Telegraph Company (NTT) in Tokyo, Japan on 1 December 1979.

The first mobile phones were still car phones, but the network was:

a cellular network with 88 cell sites with base stations, or radio towers covering all districts of Tokyo (unlike in IMTS where the network was still PSTN);

handover of the call between different cell sites was supported (unlike in IMTS where the call can only be connected to one radio mast);

automated switching without the need for human switchboard operator (unlike in ARP where the calls are manually switched). 

According to & wikipedia