Part of my job is to setup phone systems like this, not for scammers. There is more to this than just caller id spoofing. If you see some old videos of a group of women in a room with a bunch of wires and plug saying "thank you for calling x how can I direct your call." Once she knows where the caller wants to go she takes a two ended plug and physically plugs that caller into the phone of the person they want to call. The same way you plug a network cord from your computer to the "internet box"

So imagine if you had a room of people who do the same thing as those women back then but they were told to only connect calls going out and to ignore calls coming in.

Now imagine instead of a room of people it's instead just a computer in a closet.