I’m not convinced that, on balance of probability, absence of CLI indicates spam. A few of the spammers and scammers that call me do withhold CLI, but the majority just show a spoofed CLI. One trend, for a while, was to show a spoofed CLI with my own local area code. That caught me off guard a couple of times, thinking it will be one of the neighbours, only to hear a foreign accent wanting to help me to install malware on my PC for them.
In contrast, there are many valid reasons for legitimately withholding. I believe Doctors tend do so for example, because there is a risk that some people may become anxious if they reallise their Doctor had called whilst they were out, and it is too late to return the call.
I noticed it quite recently, when we were dealing with a family death. Not sure if it was the Vicar or the Undertaker, or some other relevant person (or maybe the Hospice) but I did notice they were withholding CLI.