Green’s Dictionary of Slang

telephone number n.

[the digits used in big city exchanges]

extremely large sums of money; recent use is pl.; also in phr. talking telephone numbers.

[UK]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.’.
[US]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.
[UK]J. Gosling Ghost Squad 30: Telephone Jack [...] was always broke and always talked in thousands — hence his nickname. Telephone numbers have four figures.
[US]J. Scarne Complete Guide to Gambling.
[UK]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.
[NZ]G. Newbold Big Huey 18: This guy Al was talking in telephone numbers by now, about the thousands we could make.
[UK]J. Morton Lowspeak.
[UK]T. Blacker Kill Your Darlings 236: We could be talking fucking telephone numbers here, darling.
[US]C. Stella Charlie Opera 5: Couple million in heroin is a telephone number.
[UK]J.J. Connolly Viva La Madness 33: When you start to make money in the telephone numbers category [etc.] .