Green’s Dictionary of Slang

an extremely large sum; usu. of money but see cites 1950, 1963, 1964; recent use is pl.; also in phr. talking telephone numbers.

1943		J. Phelan Letters from the Big House 141: English and American convicts have an expressive term for their conversation. They call it ‘talking telephone numbers.’.
1950		Goldin et al. DAUL 221/1: Telephone number. A fantastic sum; a long prison term. [...] Telephone number bit. Any prison term in excess of fifteen years.
1959		J. Gosling Ghost Squad 30: Telephone Jack [...] was always broke and always talked in thousands — hence his nickname. Telephone numbers have four figures.
1961		J. Scarne Complete Guide to Gambling.
1963		L. Deighton Horse Under Water (1976) 161: My pal [...] was on a tax rap at the time and it looked like he was going up the river for a telephone number.
1964		S. Woodward Paper Tiger 3: Brown, anguished by the fact that Amherst had defeated its Rose Bowl team, 7-0, the year before, ran up a telephone-number score.
1982		G. Newbold Big Huey 18: This guy Al was talking in telephone numbers by now, about the thousands we could make.
1989		J. Morton Lowspeak.
2000		T. Blacker Kill Your Darlings 236: We could be talking fucking telephone numbers here, darling.
2003		C. Stella Charlie Opera 5: Couple million in heroin is a telephone number.
2011		J.J. Connolly Viva La Madness 33: When you start to make money in the telephone numbers category [etc.] .