Boiler-room

a room in a building, ship, etc., that houses one or more steam boilers.
Slang.

a place where illicit brokers engage in high-pressure selling, over the telephone, of securities of a highly speculative nature or of dubious value.
any room or business where salespeople, bill collectors, solicitors for charitable donations, etc., conduct an intensive telephone campaign, especially in a fast-talking or intimidating manner.

Historical Examples

The Book of Cheese Charles Thom and Walter Warner Fisk
Jack Ranger’s Gun Club Clarence Young
By Right of Conquest Arthur Hornblow
The Thick of the Fray at Zeebrugge Percy F. Westerman
The Best Short Stories of 1917 Various
The Best Short Stories of 1920 Various
Jack Ranger’s Gun Club Clarence Young
Transactions of the American Society of Civil Engineers, vol. LXVIII, Sept. 1910 George C. Clarke
The Book of Cheese Charles Thom and Walter Warner Fisk
Sonnie-Boy’s People James B. Connolly

noun
any room in a building (often in the basement) that contains a boiler for central heating, etc
the part of a steam ship that houses the boilers and furnaces
the room or department in which the real work of an organization goes on unseen
(chiefly US) an office used by a team of telephone salespeople, esp of stocks and shares, operating under high pressure

a fraudulent scheme in which investors are encouraged to buy non-existent, worthless, or over-priced shares
(as modifier): a boiler-room scam

A site of illegal operations, often the telephone sale of stocks, real estate, etc, by charlatans (1930s+)
A shady and provocative gathering-place; a hotbed: the tendency of their young people to congregate in places that are boiler rooms for drugs and violence (1980s+)