A histogram with a symmetric distribution looks like this:
Leaving aside the quality of the drawing, the left and the right side of the histogram are identical if we split it by the red line. Is like having a reflexion axis.
Now, we have to discuss what of the 2 data sets may have such an histogram. This means that we must think which of them may have the same number of ocurrences for very short and very long durations, for regular to short and regular to long durations, and so on and so furth.
Pop album tracks:
In the same album we may have arround 12 tracks (small number), all of them with a duration of arround 3 minutes. We could imagine a histogram like the following:
All the songs last between 170 and 190 seconds, and one or two last a little bit more.
Phone talks:
In this case, we can think that there are some calls to say something like "mom, I'm on my way home" and other like "let me tell you the hole movie I saw yesterday night", so very short and very long calls, but they are no so common. There are a lot of "regular" calls that might last for 60 seconds.
Also, in this case, we are taking into account all the calls from people in the school in one day (yesterday), so, may be 1.000 calls? The actual number really doesn't matter, but the fact that the number is very big. Because of the big number, we can expect a symmetric distribution.
