Incoming calls should be subject to acls and default-deny policy should be practical. This means that
- caller identity should not be spoofable
- identities should form hierarchies and groups so you can allow whole organizations instead of individuals
- organizations should use predictable identities for egress calls
- most likely managing multiple identities per device is needed (e.g. personal and work identites)
etc
None of this is particularly difficult technically. Even simply slapping x509 certs on calls and having some basic filtering would achieve a lot.