Common Times between two lists

Hi all,
I have a list of call data as follows

A|B|C
Call start|Call end|Agent
08:02:06|08:09:10|John
08:03:58,08:08:02|George
08:10:19|08:20:20|Ringo
08:12:12|08:13:56|Paul
08:15:23|08:16:30|George
08:15:45|08:20:55|John

I have a second sheet where I want to know how many calls an agent was involved in, in a given 15 minute peroid (if a call spans two periods, it should be counted in both as per Ringo's call) as follows:

Period Start|Period End|John|Paul|George|Ringo
08:00:00|08:14:59|1|1|1|1
08:15:00|08:29:59|1|0|1|1

I am really struggling to find the formula that caclulcates the data.

Can anyone help?
Thanks