What are HMRC and the phone companies actually doing to prevent spoofing of CLI (presentation numbers)? I thoght it was more or less impossible to prevent a person making calls spoofing the CLI.

https://www.bbc.co.uk/news/business-48475434

I still get some unwanted calls from malformed numbers (those that couldn't possibly be a valid number) which are meant to be automatically blocked by the phone companies.

See the fair telecoms campaign take on this specific move -
HMRC stops using CLI as a means of identification - a good move, but not enough.

We also issued a more broadly based exposition of our position yesterday -
We call on HMRC, Banks, ISPs and others - Don’t open the door to scammers and nuisance callers.