There are two main ways to call functions within a macro to perform specific actions in Excel VBA:

1. Calling User-Defined Functions (UDFs):

* Create a UDF: You can define your own functions (subroutines or functions) within a VBA module. These functions can perform calculations, manipulate data, or interact with the Excel interface.
* Calling the UDF in a Macro: Within your macro code, use the function name followed by parentheses to call the UDF. You can optionally pass arguments (values or variables) within the parentheses which the UDF can use for its calculations.

Example:

```vba
' Define a UDF to calculate the sum of squares
Function SumOfSquares(values As Range) As Double
Dim total As Double
For Each cell In values
total = total + cell.Value ^ 2
Next cell
SumOfSquares = total
End Function

' Call the UDF in a macro
Sub MacroExample()
Dim dataRange As Range
Set dataRange = Range("A1:A10") ' Replace with your actual data range

Dim result As Double
result = SumOfSquares(dataRange) ' Call the UDF and store the result

MsgBox "Sum of squares: " & result
End Sub
```

2. Calling Built-in VBA Functions:

* Excel VBA comes with a rich library of built-in functions for various tasks. These functions cover areas like string manipulation, date and time calculations, mathematical operations, and more.
* Calling Built-in Functions: Similar to UDFs, use the function name followed by parentheses to call a built-in function. You can also pass arguments as needed.

Example:

```vba
Sub MacroExample2()
' Use the built-in MsgBox function to display a message
MsgBox "This is a message from the macro!"

' Use the built-in IsNumeric function to check if a value is a number
Dim value As Variant
value = InputBox("Enter a value:")

If IsNumeric(value) Then
MsgBox value & " is a numeric value."
Else
MsgBox value & " is not a numeric value."
End If
End Sub
```

Key Points:

* Remember to properly declare variables and handle potential errors when calling functions within your macros.
* UDFs offer greater customization for specific needs within your macro, while built-in functions provide pre-defined functionalities.

By effectively calling functions, you can create powerful and modular macros in Excel VBA to automate repetitive tasks and enhance your spreadsheet functionality.