For a detailed explanation of the various scams, look at the videos of 'Jim Browning' on YouTube
https://www.youtube.com/c/JimBrowning/videos
He investigates the scammers and often turns the tables on them by gaining access to the scammers' computers. The videos are interesting and distressing in equal measure, especially when they involve a vulnerable person being scammed. The scammers are completely without mercy or conscience.
The calls are able to come through on the existing phone network because the network is not sophisticated enough to screen them all out and it does actually allow caller ID presentation numbers to be changed for legitimate reasons (e.g. a business wanting to present a general switchboard number for all of its outgoing calls)
To prevent the calls coming through to your phone, you need to screen the calls rather than block them. There are various methods mentioned here
https://community.virginmedia.com/t5/Home-Phone/Landline-handset-to-block-unwanted-calls/m-p/4573945...
I have used the trueCall standalone device for a few months and it has stopped the scam calls completely from ringing my landline phone as they are screened out.