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.

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