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.)

The call did not work, that is how I noticed the issue a few weeks ago … It's a common issue with phone numbers at least in Austria and Germany while France, Switzerland and the Netherlands seems to be fine. I've never had the opposite issue, i.e., a call not working due to a missing zero after the country prefix, however, that issue might exist too. Yeah, ITU recommendations are not mandatory, that is how the ITU works, and there has been no need to make them mandatory because they are widely adopted, including E.164, also due to ITU's status as a UN organization.