I had some of these - I've attributed them to the "fine" MJ network that failed to complete the call. In fact I had a couple of these with apparently no one on the other end but later I found out (when the other end called my Verizon land-line number) that he could hear us but we couldn't hear him.
That's the nature of using a service that travels over the unreliable-by-design public internet. Packets go astray. Sometimes they are Call Setup packets. It is not a phenomenon that is particular to MJ.