decrease or decreases?

On the one hand, there were three decreases. On the other hand, I seem to treat them as one single decrease in the first sentence. Therefore I don't know whether I should use decrease or decreases in the second sentence. Please advise.

This then rose to a maximum of 90 billion minutes in 1999 before falling back to its original level in 1995. Despite the decrease/decreases, this type of call was more common than the other two types throughout the entire period.

Use "decrease" to match your description. Remember, what you should be describing is what the data represents, not the form it is presented in. This does not mean that you cannot say that the number of local calls decreased in each of the three years 2000-2002, and then use "decreases", but don't feel that you have to describe the shape of the chart. If the data had been plotted on a line graph rather than a bar chart, then it would most likely show a gentle hump, with a peak in 1999 and no steps, and you could still describe it in exactly the same way, whether as a single falling back or as decreases in three successive years.