You can use any of the 800 series routers to connect back to the office, you just need to ensure that the IPSEC tunnel is setup at the (A) end where the UC-520 is, or you set it up on the UC-520 itself.
You then need to make sure that 800 series router has all the correct programming on it, for instance the IPSEC tunnel and also the ACL's set in place
the 7961 phone will need a power pack unless you are going to put a small 8 port PoE switch at the teleworkers end, and you will need to manually program the TFTP server address into the phone.
Alternatively you can use the SR-500 series, in particular the SR-520 seems to be the most chosen one, this is a good fit and can also be programmed using CCA.
Given the advancements that have been made with the UC's and the other paired or compatible devices that work with the UC-500 series, it almost makes common sense to do a rebuild of your system using CCA (Latest version) and upgrade the system to the latest Software Pack, this way you can use CCA to manage the system and also correctly configure the remote teleworker devices properly
In your position the SPA-525G2 would be ideal as CCA auto provisions the phone if you tick the remote teleworker check box, it is handy and just works.
The things you need to remember are:
- Setup IPSEC on the UC-500 (EZ_VPN)
- Setup IPSEC on the Teleworkers router (EZ_VPN)
- Setup ACL's on both the UC-500 and the Teleworker Router
As Steve mentions you should read over the technical enablement lab, you can go HERE to look through them
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