The Cisco ATA-186 you have from Vonage has 128Bit encryption, and cannot be unlocked unless you hooked a Cray Supercomputer up to it and had it spend a few months cracking the encryption.
You can find good deals on ATA devices here:
http://www.VOIPSupply.com – Online reseller of VOIP Equipment from leading vendors including Cisco Systems, Grandstream, Sipura, Snom, Mediatrix, Audicodes, Digium, Carrier Access Corp and more