I've never heard of someone trying to setup two UC500's with VPN using 1 ISDN. You can, however, use an 871 or 500 series security router to VPN back to the UC. The UC500 would perform call processing for the remote site. I believe (unless it's changed) that you can have up to 5 phones behind the 871/500.
You can also VPN two UC500's together to use for 3 or 4 digit site to site calling. This is called a multisite deployment.
I would not use UC5xx for that type of deployment. I would move up to CME or Business Edition. The UC5xx wasn't meant to be deployed in that manner. I'm not saying it can't be done, I'm just saying that I wouldn't advise it.
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