There is something wrong in this question: you specify a ratio with three numbers, while only two are expected. I guess the second ":" should really be a decimal point, like this.

1 : 3.5

If not, just follow the reasoning with the correct figures yourself.

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)

So, in order to make 23 sales, you can expect to make 23 x 3.5 calls, i.e. 80.5

Now that is not a round number, so to be more certain you get the 23 sales, you need to round it upwards.

Answer: 81 calls