If you have FileMaker 11 or newer you can use portal setup to add a portal filter:
Shifts::ShiftStart < Calls::TimeStamp AND Shifts::ShiftEnd > Calls::TimeStamp
With all recent versions of FileMaker you can also include the above fields as match fields in your relationship with the same inequality operators shown in the above portal filter expression.
Note another option might be to log a ShiftID in the calls table each time a record is added to that table. Then you could use the relationship:
Shifts::__pkShiftID = Calls::_fkShiftID--a relationship that will update a bit faster in your portal than will one using inequality operators.
Awesome... thanks so much. :)