that's something I've seen lately, but I don't know why we're adding  "carrier-unknown" when we don't have it ?

I've just checked the SMS/MMS UX design. There's nothing about the carrier line in the MMS spec, so I suppose we should take what's inside the SMS UX design, that is in https://mozilla.box.com/s/wzgsb3lkqglv0dnfdgzs/1/864518456/7994809160/1. In page 6, annotation 10, I think it's written we're supposed to show nothing if the carrier if it's unknown, and we're supposed to hide this line if the phone has no contact.

I think that's how it used to work in 1.0.1, I don't know why this was changed.