[Steve Van Doorn]

If you look at the site Monkey is using. In the far upper left hand side there is the call sign and the tail number. (Monkey has the date covering the tail number in the upper left.) On the lower right he highlights the tail number of Xiden's plane. in the call sign it also shows the tail number. Given that the call sign is N/A it uses the tail number as a default I believe.
Here is the site so you can see what he is looking at
https://globe.adsbexchange.com/

[Drago]

“Callsign” is not registration (”N number”) or “Flight #”...those “flight tracker” sites (”Flight Aware”, “FlightRadar24”, “ADSBexchange.com”(his) just use ADS-B info. to track flights... ADS-B includes “tail#/N-number” or “flight number”. ATC and pilots use “callsigns” for radio comms...things like “Speedbird” (British Airways), “Critter” (ValuJet), “Cactus” (certain US Air flights in the past, i.e. “Miracle on the Hudson” (inherited from America West Airlines from before the merger(s)) and “Air Force One”. ADS-B can be turned off all together, or the “tail #” part can be obfuscated for privacy/security. Military flights do it frequently for “OpSec”. The “ADSBexchange” ADS-B site probably just puts “N/A” in there when the ADS-B ground station receiving signal is weak or the aircraft is in “privacy mode”. (Like a lot of the Epstein flights to “Pleasure Island” were).

https://www.faa.gov/nextgen/programs/adsb/faq/

https://www.airspacemag.com/daily-planet/cactus-speedbird-and-other-great-call-signs-180958518/

https://aerosavvy.com/airline-call-signs/?doing_wp_cron=1616402172.9800240993499755859375