Hi Gennaro,
Is CallID the same as a Ticket? Unfortunately your sample data doesn't reflect the schema you listed for us.
I am assuming you are grouping on CallLog.CallID. Are there any groups before or after that?
Your formula looks correct so that leaves the question of how your report is grouping the data and if CallLog.CallID can have more than 1 assignment.
Would a ticket with multiple assigments like:
Open Date Ticket # 1st Level Assignee Date Assign Ticket Owner Group Name
2013-12-01 1125474 Mel Limm 2013-12-01 LIMM01 BWHelpDesk
2013-12-02 1125474 John Doe 2013-12-01 JDOE01 BWHelpDesk
If you are grouping on CallID, in the group header or footer drop a formula that just does a GroupCount so you can see what the count actually is. If you see the count properly then the formula should work.
Thanks,
Brian