From what I’ve gathered from your post it looks like you want to encrypt the voice from the phone itself. I don’t think that feature is available in the phone itself. I would suggest using softphone with the vpn client software and encrypt the IP that way. Or if the first hop isn’t an issue, at least encrypt it from the gateway to the remote site or PSTN. I haven’t tried it myself but maybe someone else here has.
Hope this helps.
