Acredito que seja isso mesmo conforme pesquisa feita.

“Many businesses continue to use PBX lines, and although they still work, it doesn’t make them secure. PBX lines create several branches off of the main phone line, and in many cases are routing voice traffic over internet connections, which results in a severe lack of security.” - https://www.altigen.com/phones-systems-that-arent-secure/

“ The phone setup i have now has main phone line branching off to feed all other phones and basement computer so will one filters at the beginning before the branches take care of all the regular phones lines?” - https://www.dell.com/community/Networking-Internet-Bluetooth...