All books of accounts and summary records shall be preserved for not less than three years. Billings, sales slips and other memoranda regarding sales made and services rendered, as, for example, the individual slips prepared in connection with telephone calls, need be kept no longer than they are usually kept by the utilities, but the summaries or recapitulations thereof shall be preserved for three years.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 47.3