Hello
You have a very short formula for a relatively long problem.

To simplify it, can we agree to break up the 24 hour cycle into the 15 minute intervals ? This would create 4 x 24 = 96 time blocks. I would then determine which block each call record would fall into, some into multiple.

At the end it is a matter of summing up the number of calls per block to give your concurrent calls.

Do you want to give it a try ? You could run it through formula without VBA and being familiar with array formula, you could probably manage that. If you get stuck, shout !

Regards