It's pretty much impossible to fix. The reason is that anyone who has a trunk line, e.g. a T1 PRI, has, and it should be noted, has always had, the ability to set their caller id to whatever they want. Better yet, they have also always had the ability to read caller id even from those who have blocked their callerid. This functionality is just part of what you get with a trunk line. The reason for it is due to the nature of how PRI works, where there is not a 1 to 1 mapping of numbers to "lines." Therefore the originator of a call on one of these lines needs to be able to say what the callerid should be for the call. Companies have all sorts of legitmatite uses for this sort of thing, even the ability to set the caller id to numbers other than what is technically on their account. For example if I'm calling from my office, it might be nice to be able to make it look like the call was coming from my cell phone, so that I get calls back on that from people that use caller id.

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