Initial Response field

Hello,

I have a business requirement to start tracking SLA on the case level.  I am trying to set up a case field called "Initial Response" that will pull the "Start Time" from the first activity where the Type = 'Portal Note' or the Interaction (ActivitySubType) = 'Outbound Call'.

Currently, I'm unable to get the Start Time to pull to Initial Response on a case, and this is how it is set up:

"Initial Response" is a Case-level Date\Time field with the following field validation:  IIf((JoinFieldValue('<Activity>', [<ServiceRequestId>], '<Type>')='Portal Note' OR JoinFieldValue('<Activity>', [<ServiceRequestId>], '<ActivitySubType>')='Outbound Call'), JoinFieldValue('<Activity>', [<ServiceRequestId>], '<StartTime>'), DO NOTHING)