The modal class is the interval containing the highest frequency of calls.

To approximate the mean call duration from grouped data, one must calculate the weighted average of the class midpoints, using the frequency of each class as the weights.

The exact median call duration can always be determined from the grouped frequency table without knowing the individual call durations.

If the distribution of call durations is highly skewed with a few extremely long calls (e.g., $120$ minutes), the mean will be more affected than the median.