As far as I know the system does not currently support T.38 termination (though I believe it is able to pass T.38 traffic).
Fax over SIP using G.711 works well in my experience... IF and only IF there are not latency/loss/jitter issues with your internet connection. We have some people who have been using it for a long time with no issues, and others that go from working to 100% failure overnight due to ISP issues. (all this is of course assuming you are talking about SIP over the public internet)
I can't speak to Cisco's plans for the future. However you might want to check CME... UC500 is based on CME but is usually a few versions behind the latest CME release... if it exists in a newer CME, it will almost certainly be added to UC500 at some point.
I realize I should clarify something, this is a pure CLI config. You can't do this from CCA.
Anyway in CLI:
Voice service voip
Fax protocol t38
No sip-profile 1000
As for the "No sip-profile 1000" line, this was because I found that CCA (I assume because I didn't put it there) put in a SIP profile 1000 that was stripping out a required SIP header. You might not need this.
The otherwise, you also need a SIP provider that supports T.38 if you are going to be faxing to anybody outside of your system.
I'm not sure how this specifically would work with your Site 2. Is all phone service to Site 2 via the VPN to Site 1? Is the fax machine at Site 2 analog? How is it currently connected?
Just be aware if you do the above CLIconfig it will put your system out of scope, if you cannot do it in CCA and you are not UC-Express Certified then you are pretty much on your own (In theory).
However the UC-500 supports Fax over SIP or "FoIP" quite well, its tolerance level is reasonable or much better than say the Linksys PAP devices and I have seen it operate for a while with only minimal issues, although there have been the occasions where it wouldn't work at all and just give heaps of grief.
Remember if you cannot do something in CCA and you are not certified for CLI work around's, then avoid selling the solution and try and keep the system away from it, it could cost you the deal
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