how to reject INVITE from unknown sources

[sergeysi]

Is there a way to configure an IP-phone to reject INVITE messages from unknown sources and/or accept them only from servers at which a telephone is registered?

Here is an example of problem. We have a couple of phones with "white" external IP addresses. Script-kiddies find open 5060 UDP port and send INVITE messages, telephone rings. This is very annoying. We currently don't have possibility to put router/firewall before IP-phones. Is there a way to block INVITE messages which do not come from SIP-proxy?

I think it works on all phones but I tried to simulate it on T20P and T22P.