Give hint in error message.

cfp/models.py

@@ -492,7 +492,7 @@ def validate_phone_number(phone_number: str):
         raise ValidationError(str(err))
     else:
         if not phonenumbers.is_valid_number(number):
-            raise ValidationError(_("Invalid phone number"))
+            raise ValidationError(_("Invalid phone number, try using the country code (like +33 for France)"))
 
 
 class Volunteer(PonyConfModel):