There is no way for you to verify whether the caller's phone number is actually the real source. Anyone can impersonate anyone. It's like the source address in an IP packet. You can put whatever you want in there.

This is mostly a US problem though. In Europe, the operators tend to be more strict on this, AFAIK.

You should read the book "Ghost in the Wires" by Kevin Mitnick. It will really open your eyes as to what is possible with phones.

can I borrow your copy? I will pay shipping to and fro' :)

Sure! But honestly, it will probably be cheaper for you to just buy it from Amazon for $8. Or, read it for free using Overdrive (if you have a Kindle/ereader).

But this way I get to bug you about my discoveries and compare interpretations with yours... the knowledge comes with instruction hehe