nothing personal, but it just sounds like the data entry isn't consistent, you know? I mean, it doesn't make sense that the same call would be handled on two different account numbers.
So, the only solution I can really pose at this point will give you a summation of "down time" for each account. Here's what you do. Create a formula:
{@Revisit Time Difference}:
if (Previous({CALL.CALL_NUMBER}) = {CALL.CALL_NUMBER}) then
(
if (Previous({THREAD_EVENT.THREAD_EVENT_VISIT_NUMBER}) = 1)
then dateDiff("n", Previous({THREAD_EVENT.STOP_DATE_TIME}), {THREAD_EVENT.START_DATE_TIME})/60
else 0
) else 0
------------------------------------------------------------
Put this formula into the details section of the report and suppress it.
Next, right click the formula and click "Insert Summary." Choose the appropriate group for the summary and then make sure the box "Insert summary for all groups" is checked. Click OK and you are good to go.
Like I said, this won't allow you to compare individual calls on different accounts w/ the same call number, but it might be close enough to help out. Otherwise you are gonna have to go the route of SQL expressions, and even then, depending on how your CSR's enter the data, it still might not work.