Its a common feature for PBX'es to rewrite their outgoing caller ID on forwarded calls to match the origin caller ID. Say you've got an office desk phone that you have set to forward to your cell phone while you're out. Someone calls your desk phone, it forwards the call to your cell phone, what caller ID should be displayed? Technically the call to your phone is coming through your desk phone (well, your office's PBX), but doing that would mask who is actually calling. So the PBX rewrites its caller ID info to appear to be as the origin when it calls your cell phone.

This is technically spoofing caller ID, but is clearly not fraud.