OK.
1) Create a group on the Ticket Number field
2) Create a formula that converts the CallLog.ModDate to date. Search the forum and there are many examples on how to convert a string to date
3) Go to the Record Sort Expert > Add the above formula to the Sort List and sort in Descending Order
4) Create a formula (@Owner) with this code:
{CallLog.Tracker} = {Assignment.Assignee}
5) Place this on the Group Header and Suppress it. Then go to the Selection Formulas > Group and use this code:
{@Owner}
-Abhilash