Table with 'blanks' and calculating 'Coverage' based on customers, calls and time

Hi Raketenrudi 

Try these measures

Last Call Date = 
VAR CustomerID = SELECTEDVALUE(Customer[Customer ID])
VAR MaxMonthNumber = 
    CALCULATE(
        MAX(Calls[MonthNumber]),
        FILTER(
            ALLSELECTED(Calls),
            Calls[Customer ID] = CustomerID
        )
    )
VAR LastMonthName = 
    SWITCH(
        MaxMonthNumber,
        1, "JAN", 2, "FEB", 3, "MAR", 4, "APR",
        5, "MAY", 6, "JUN", 7, "JUL", 8, "AUG",
        9, "SEP", 10, "OCT", 11, "NOV", 12, "DEC",
        "нет"
    )
RETURN
    IF(ISBLANK(MaxMonthNumber) || MaxMonthNumber = 0, "none", LastMonthName)

Call Count = 
VAR CustomerID = SELECTEDVALUE(Customer[Customer ID])
VAR Result = 
    CALCULATE(
        DISTINCTCOUNT(Calls[Call ID]),
        FILTER(
            ALLSELECTED(Calls),
            Calls[Customer ID] = CustomerID
        )
    )
RETURN
    IF(ISBLANK(Result), 0, Result)

Coverage = 
VAR CustomersWithCalls = 
    CALCULATE(
        DISTINCTCOUNT(Customer[Customer ID]),
        FILTER(
            Customer,
            [Call Count] > 0
        )
    )
VAR TotalCustomers = 
    DISTINCTCOUNT(Customer[Customer ID])
RETURN
    DIVIDE(CustomersWithCalls, TotalCustomers, 0)

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Thank you.