The number in column S tells you how many calls overlap with the call in that row. This is a time range (start to finish)
If you want to know how many calls are ongoing at the same time as a specific time (i.e. the start time)
=COUNTIFS($Q$2:$Q$101,">="&E2,$E$2:$E$101,"<="&E2)
For end time
=COUNTIFS($Q$2:$Q$101,">="&Q2,$E$2:$E$101,"<="&Q2)
Does that help?