The problem is rather loosely defined. My guess was based on...
Times, generally range from one minute to less than two minutes which gives a 59 minute range.
Therefore there is no chance of error for numbers greater than 119.
One minute would normally be called as such.
So errors could only be made on calls from 101 to 119
If calls are randomly inconsistent about half in the range of 101-119 could be misconstrued.
So thats 1/2 of 19 divided by 59. Strictly speaking, 9 and a half, divided by 59. Somewhat less than 1/6th.