[amsat-bb] iss packet invalid callsign nnnnnn

hi folks, just looking at the iss mheard callsign list I see one called nnnnnnn. Does anyone know what or who this is? maybe just a silly error, or maybe someone is using the iss without a call. we hams police our own if I'm not mistaken. anyway why its here to observe and go over hows about some explaining to us why there are serveral calls for the iss. I see in the list, ISS, ISS-10,ISS-5, RSOISS-4, -4 I Think is the heard digi, but always wondering what the other iss calls are. If anyone knows please post, thank you much, kc8gkf-5 UISS User.

attached is the isscallsign heard list