SIP message, Via, Call-ID and Contact headers

        In a SIP message, how is the "Via" header set?
If the value that appears in that header doesn't look correct, how can it be altered using Cisco command(s)? Same questions for the "Call-ID" header, the "Contact" header. What if I have an ISP (for Internet) and a different ITSP (for telephony) and the IP of the ISP appears in those SIP headers? I can tell that phone calls to and from the PSTN fail. But I don't know why, this is why I'm asking these questions. Please help. Thanks.

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