count frequency

Dear Expert
I 've attached excel file about tel number by customer who complain as by date in column.
I need to know that which number have most complain frequency by different of date such as complain on 1 sep and then 5 sep = 4 date diff.
frequency of customer to complain such as find date diff of customer per each that there are how many of day to call to complain if yes so they face the problem often.

how i can use which formula?
please kindly advised.
thanks and br.

attached file.