Well, at some point this car had a hands-free installation for phone, so that makes sense.
Although that is an unusual place to have a connector for that (I think), and my question arose because it was not the first time I found unused connectors. E.g.: for the rear speakers and trunk light, that my car did not have stock (a blank piece of plastic covered the holes instead). Actually I found this connector while I was checking if I had factory wiring for the trunk light.