Without seeing your code it is impossible to guess what the problem is, however adding some error handling should indicate the problem. Select a message and run the following

Sub Test()
'Graham Mayor - https://www.gmayor.com - Last updated - 31 Oct 2020 
Dim oItem As Object
Dim PhoneNumberString As String

    On Error Resume Next
    Select Case Outlook.Application.ActiveWindow.Class
        Case olInspector
            Set oItem = ActiveInspector.currentItem
        Case olExplorer
            Set oItem = Application.ActiveExplorer.Selection.Item(1)
    End Select

    If TypeName(oItem) = "MailItem" Then
        If InStr(1, oItem.Subject, "Voicemail received from") > 0 Then
            PhoneNumberString = oItem.Subject
            PhoneNumberString = Mid(PhoneNumberString, InStr(1, PhoneNumberString, "("))
            PhoneNumberString = Left(PhoneNumberString, InStr(2, PhoneNumberString, "(") - 1)
            Beep
            MsgBox PhoneNumberString
        Else
            MsgBox oItem.Subject & vbCr & "is not a voicemail message", vbCritical
        End If
    Else
        MsgBox "Not a mail item!", vbCritical
    End If

lbl_Exit:
    Set oItem = Nothing
    Exit Sub
End Sub