Phone numbers written in the international format with country code are not valid if the zero from the national prefix remains. An example is the telephone number for EDTF in Germany, at least if you use iOS:

+49 0761 509626 is not valid, +49 761 509626 is valid. (The German phone number is 0761 509626.)

As it happens, we are given telephone numbers for German airfields digitally so we can probably help with this. Many of the telephone numbers we are given do not have the +49 international prefix so we add it. If when adding it, it would also be helpful if we then removed a leading zero, we can do that.