As Rahul HCM indicated, take out the + from the number. The standard validation SAP is doing on Telephone and Fax number fields is simply different. If you look at the F1 help, it reads:
'If an + is entered as the first character, the system checks whether the specified number starts with a country code. If so, a warning message is displayed because the country code is automatically determined by the system and should not be stored in the Telephone number (Fax number) field.'
Hope this helps clarify!