A close ratio of 1 : 3.5 means that on average you close 1 deal in every 3.5 phone calls (or better to understand: on average you need 7 phone calls to close 2 deals)

In order to make 23 sales, you can expect to make 23 x 3.5 = 80.5 calls.

80.5 is not an integer, so to be more certain you get the 23 sales, you need to round it upwards thus the answer is 81 calls.