Make extension reachable from inside only

[orders@boxtechs.com]

We have a couple of generic SIP door phones on our 6X, which we have connected to our door entry system. They work great, except that the extensions of the generic devices can be reached from the outside world, which is a security hole given that the doors can then be unlocked. So to fix this, we simply set these extensions to disallow incoming calls. It fixes the security hole, but it causes an inconvenience, as we would sometimes like to call the extension and open the door if we know someone is on their way to the door from the outside.

What we would like to be able to do is configure these extensions so that they can only be called from other extensions on the phone system, but cannot be reached when someone calls in from the outside world. Is this possible?