This or a similar message you will receive if you manually format your contacts by FormatNumbers and then change a still selected contact.

This message appears while saving a contact. This problem you can avoid if you select another contact before and then switch back to the orignal one. Sometimes you have to change a different folder. If both will not work please restart Outlook® (should normally no need for to do).

By a new entry in the registry you can change the behaviour of the program and avoid this error message. The effect is, that you may have problems to work with all formats (please see the article Why are the call numbers not converted (correctly)?).

But in most of the cases this will not have an effect, particularly not if you work with the international format (+49 (89) 12345678). Please create a new String under HKEY_CURRENT_USER\Software\VB and VBA Program Settings\ESM-Tools\FormatNumbers\General and name it NoRDO and give them the value True.