[Serusers] Call-ID discrepancy

Hi All, I notice during the NGREP session , there is this field called Call-ID . It is something like this format :- 9ce97fd3-1a800040(a)172.16.1.123 <mailto:9ce97fd3-1a800040@172.16.1.123> . According to the RFC , it is said that this field is generated by the UA who started the request. I was just wondering, how did they get the IP Address at the back ? Which IP Address did they reference it from ? should use the same Call-ID. When they say this, does it mean the entire field with the ip address (eg. 9ce97fd3-1a800040(a)172.16.1.123 ) or just the front part of it (eg. 9ce97fd3-1a800040 ) ? I notice that during a dialog session from a UA to the server going to the PSTN gateway, it was refering to this Call-ID until the point when the request was forwarded to the PSTN gateway. When the PSTN gateway replies, it replied with a same Call-ID with a different IP address at the back (9ce97fd3-1a800040(a)123.234.123.234 ) 172.16.1.123 - Private address of the UA 123.234.123.234 - Public address of the UA Strange ? Give your comment. Somehow i feel that this implementation is wrong. Regards, Sam