Fake "from" addresses are trivially easy to create.
The analogy I like it that it is exactly as easy as writing a fake return address on the back on an envelope and popping it in the post.
Similarly, phone caller IDs can be faked by anyone with their own internal phone system. I am not sure about those with plain copper lines but essentially any multi-line business using digital lines (ISDN) gets to put their own caller id on the call. This may sound odd but consider the case where for reliability a business buys lines from more than one supplier yet wants to put the corporate 0800 number on the wires as the caller ID. The phone companies allow that. The line user gets to put on any caller ID that they choose.