Calculating concurrent calls in a call center

Hello,
I'm editing the original post because I didn't include a dummy spreadsheet and enough details. My request was too complicated and I'm looking for a simpler solution. Hopefully someone can help me on this one. My goal is to:

1. Calculate how many concurrent calls a call center has at any given time within a 24 hour period. If at least two calls overlap in start & end time, then it should be counted as two concurrent calls.

I have 3 columns of data: Date, End time, and call duration in seconds. (this is all I have to work with)

I've tried using hamjam's code but I don't have enough expertise or the required information to make it work.

Reason for this spreadsheet: I have a VoIP service provider that limits the number of concurrent calls I can make. I need to determine if more capacity needs to be purchased.