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Is it possible to get the 7921 IP phones registering to CCME through NON Cisco APs?
Is it also possible to register remote 7921 through a VPN?
Yes can register 7921 through 3rd party APs, but only Cisco APs are supported and guarantee 100% interoperability with the 7921G phone.
If the 7921G is behind a hardware VPN, then yes should have no problems connecting to the CME assuming there is connectivity.
thanks. Do you know a document where I can find the most important details for these two issues?
I think disable cdp should be the very first step, something else?
We don't support 3rd party APs, therefore no docs available.
For ports used by phones and CM, see the follwoing url.
7921G Deployment Guide