Starting with firmware version 8.4.27, all snom firmware versions for all snom phones with the exception of the D120 & D305 include the ability to build secure VoIP-Infrastructures via OpenVPN-Technology.
To be able to use the VPN client on your Snom phone, download the patch here from the Firmware Update Center.
This means that almost every current snom phone could have a built-in VPN client.
Further information about the configuration can be found here: VPN Support of Snom Phones
Of course, they also need a VPN server.