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