That's the sad thing about caller ID; it's trivial to spoof. I could easily do it with our office phone system. We have 8 lines and thus 8 numbers, but it doesn't matter which line the system uses when I make an outgoing call, the call appears to come from our main number (267-638-2300) because that's how I have it programmed. I could just as easily program it to look like it's coming from 843-023-5016.
The way this is abused is quite annoying. Most spam calls to my cell phone appear to come from the same area code (267-xxx-xxxx) as my cell, and quite often, even the same Exchange (267-421-xxxx). I think the spammers do this because they figure you are more likely to answer a number that's "close" to your own than one from an someplace you don't recognize.
The spam calls have gotten annoying enough that I (and many others) have taken the approach: If you call my cell phone from a number that's not in my contact list, I will not answer; talk to my voice mail. Of course the spammers usually won't leave a message.
