Why do telephone numbers that are NOT in service show on my phone's caller ID?

I'm using internet voice over IP phone service (VoIP) from ITalkBB. Sometimes I call back the unrecognized phone numbers and get the message of ';I'm sorry, the number you have reached is not in service.'; I suspect the calls from these numbers are from telemarketing agents. No matter who they are but how do they use a bogus phone number? Or are there any explanations about this? - I don't think it's my phone's problem because I have three phones with caller IDs and all of them show the same number.Why do telephone numbers that are NOT in service show on my phone's caller ID?
It could be spoofed, or they could be using calling cards.Why do telephone numbers that are NOT in service show on my phone's caller ID?
I get this same condition on a regular phone also The people that called you spoofed the number shown There is no way of finding out the real number called





Bob