Why: Microsoft Outlook – This is not a valid number
Did you receive a prompt while working in Microsoft Outlook? The message reads This is not a valid number. In most cases, you were typing a message in Outlook and the dialog box appeared. You click OK and the box disappears.
There is no known knowledge base article on Microsoft’s website for this error message, however others that reported it also said that clicking OK made the box go away and didn’t receive it again after.
If you received this message or continue receiving it after clicking OK, please let us know below the steps you did to reproduce the message.