had separate firmware on a 4610 once for doing just this..
Although.. never got it going,as it was only supported and worked on certain VPN Security gateways.
The 4610 booted to a choice menu right at the start, VPN or not VPN.
To be fair, we knew nothing about VPN, or the appliances that were supported, so it got shelved. We just make the routers setup the tunnel and use normal IP phones.
routers with VPN support are cheap enough now too.
Check this link to the H323 PDF
page 44 is the VPN info...
it says there should be separate firmware on the Admin DVD for each of the different phones..
1.IP Office VPNremote Phone Firmware
This firmware is provided on the IP Office Administrator Applications DVD for IP Office 4.1 and higher. IP Office
VPNremote firmware is provided for the 4610SW, 4621SW, 5610SW and 5621SW phones and 9600 Series phones
only. Other VPNremote phones are not supported.
and release 6 doesn't require a VPN extension license now.
Hope that helps some?
Cheers,
Chris
Auckland NZ