I found documentation about how to Add custom shipping carrier validations. This doc explains how to create from scratch a brand new validator and its own validation rules and how to register them and how to add them in the checkout layout etc. etc...
The point is : I don't need to create a new validator from scratch. The carrier already exists, its validator too. I just want to add an extra validation rule to the existing ones (and fyi, I want this carrier validation to fail if shipping address telephone is not a mobile phone).
Is there a way to inject my own rule in the existing ones (with a mixin maybe ?) or do I have to go through the full creation as described in the official doc, to end up pushing an additional validator for this carrier ?