Mathematica naturally handles this type of problem smoothly with concise code. A quick way is to create a count function "f" that counts one customer if he or she is on phone during the start time to end time. Otherwise do not count.

Here is the implementation:
Now lets work on a set of numbers: 
So the first customer called in for refund at time point 1 and your customer service guy successfully turned him or her down at time 2. Similar things happened to the second and the third.
Lets visualize this process. At any moment the number of customers online is: (Map- /@- the f over a list)
Therefore, as we insert everything into one place, the plot funciton looks like

Now we have some persistant customers who really want their money back. Assume we have 20 such customers and calls repeated to dispute. The number of call events may vary (though 50 is set, but Union will remove the duplications in undertermined way)
For the 1 customer, whose events are recorded in timeStamp[[1]], called in at time 6 and hold off at 7. Then he called again  at 12 and stopped at 13 because of the annoying music in the phone. So on so forth. Same thing applies to other customers. The profile of each customer is visaulized in the following way: 

Using a very similar code to the individual case, you can plot for the whole group: 

Code:
preprocessings Visualization: