If you mean the +37 dialing code, it belonged to the German Democratic Republic. The country code +37 was finally withdrawn in April of 1992.
The German Democratic Republic (East Germany) was established in 1949 from the area of Germany occupied by the Soviet Union. East Germany was proclaimed in East Berlin on October 7th, 1949. It was declared fully sovereign in 1954, but Soviet troops remained on grounds of the four-power Potsdam agreement, largely to counteract the American presence in West Germany during the Cold War.
East Germany was a member of the Warsaw Pact. Following free elections, it merged with West Germany to form a united Germany in 1990.