Rewrite a function to read a list from external file

Hello Excel Forum!

I have a macro that uses the following function.

Function FlagDep(AcctNo As String)

'   Check if an account number is flagged for
'   special handling

    If AcctNo = "" Then GoTo FlagDep_Err
    
Select Case AcctNo
    Case "1234", "12345", "123456"
        MsgBox "Account number " & AcctNo & " needs special handling. Check account notes."
    Case Else
End Select
Exit Function
    
FlagDep_Err:

    MsgBox "No account number specified."

End Function

The way I am calling the function is like this:

    For Each cell In Range("A1:A" & Range("B1").CurrentRegion.Rows.Count)
        If Not cell.Value > "0" Then
            MsgBox "You're not done entering account numbers"
            GoTo VeryEnd
        Else
            If DepReady = True Then FlagDep (cell.Value)
        End If
    Next cell

I need to rewrite this function to make it more portable with other people.

Instead of getting the list of flagged account numbers from Select Case, I need it to pull the list from an external file. Let's call the file Z:\Flagged.csv for the sake of convenience. The file will be in .CSV format. Account numbers that need to trigger the MsgBox will be in the A column, one account number per row like in my example: Flagged.csv. There will be data in the B column that needs to be ignored.

Is there a simple way to do this? I suspect there is but I've never worked with making a macro read data from another file and my search results are proving confusing.

Thank you in advance for any help that you can offer!