Add New Dynamically Updating Column To Data

BeckiB , you might have to play with the measures according to your model, but here are the ones I used:

MinCallStartDateTime = MIN ( 'Date'[Date] )

MaxCallStartDateTime = MAX ( 'Date'[Date] )

MaxCallsMade = 
VAR mn = [MinCallStartDateTime]
VAR mx = [MaxCallStartDateTime]
VAR tableWithinDates =
    CALCULATETABLE (
        VALUES ( CallCent[CallerNumber] ),
        FILTER (
            ALL ( CallCent ),
            CallCent[StartDateTime] >= mn && CallCent[StartDateTime] <= mx
        )
    )
RETURN
    MAXX ( tableWithinDates, CALCULATE ( COUNT ( CallCent[CallerNumber] ) ) )

FirstCall = 
VAR mn = [MinCallStartDateTime]
VAR mx = [MaxCallStartDateTime]
RETURN
    CALCULATE ( MIN ( CallCent[StartDateTime] ), ALL( CallCent[StartDateTime] ), CallCent[StartDateTime] >= mn && CallCent[StartDateTime] <= mx )

FirstCallAgent =
VAR firstCall = [FirstCall]
VAR t = FILTER ( CallCent, CallCent[StartDateTime] = FirstCall )
RETURN
    MAXX ( t, [AgentName] )

CallTally =
VAR mn = [MinCallStartDateTime]
VAR mx = [MaxCallStartDateTime]
RETURN
    CALCULATE (
        COUNT ( CallCent[CallerNumber] ),
        ALL ( CallCent ),
        CallCent[StartDateTime] >= mn,
        CallCent[StartDateTime] <= mx,
        CallCent[StartDateTime] <= MAX ( CallCent[StartDateTime] ),
        VALUES ( CallCent[CallerNumber] )
    )

RepeatCallPerc =
VAR t =
    FILTER (
        ADDCOLUMNS (
            SUMMARIZE (
                CallCent,
                CallCent[StartDateTime],
                CallCent[AgentName],
                CallCent[CallerNumber]
            ),
            "CallTally", [CallTally],
            "MaxCallsMade", [CallsPerCaller]
        ),
        [CallTally] = 1 && [CallTally] <> [MaxCallsMade]
    )
VAR rows_first = COUNTROWS ( FILTER ( CallCent, [CallTally] = 1 ) )
VAR rows_repeat = COUNTROWS ( t )
RETURN
    DIVIDE ( rows_repeat, rows_first, 0 )

LastCallAgent =
VAR mn = [MinCallStartDateTime]
VAR mx = [MaxCallStartDateTime]
VAR caller = MAX ( CallCent[CallerNumber] )
VAR maxDate =
    CALCULATE (
        MAX ( CallCent[StartDateTime] ),
        CallCent[CallerNumber] = caller,
        CallCent[StartDateTime] >= mn && CallCent[StartDateTime] <= mx
    )
RETURN
    CALCULATE ( MAX ( CallCent[AgentName] ), CallCent[StartDateTime] = maxDate )

LastCallAgentPerc =
VAR mn = [MinCallStartDateTime]
VAR mx = [MaxCallStartDateTime]
VAR t =
    FILTER (
        ADDCOLUMNS (
            SUMMARIZE (
                CallCent,
                CallCent[StartDateTime],
                CallCent[AgentName],
                CallCent[CallerNumber]
            ),
            "@maxDt",
                VAR lastDt =
                    CALCULATE (
                        MAX ( CallCent[StartDateTime] ),
                        ALL ( CallCent[StartDateTime], CallCent[AgentName] )
                    )
                RETURN
                    lastDt
        ),
        [StartDateTime] = [@maxDt] && [StartDateTime] >= mn && [StartDateTime] <= mx
    )
VAR allCallersCalls =
    CALCULATE (
        COUNTROWS ( VALUES ( CallCent[CallerNumber] ) ),
        ALL ( CallCent ),
        CallCent[StartDateTime] >= mn && CallCent[StartDateTime] <= mx
    )
VAR currentAgentLastCalls = COUNTROWS ( t )
RETURN
    currentAgentLastCalls / allCallersCalls