Plus if it didn't work well/too cumbersome/etc with email, it probably won't be any better with voice. We have lots of experience with what doesn't work for email.
I sort of figured that the shaken/stir model that ( i happened to propose in their first meeting) of: "get the originator (handset, ebony phone, call-warehouse) to digitally sign the call initiation, propagate that through the network to the receiver (so they could associate the md5/sha256/cert-signature/etc with an identity, and let the receivers decide: 'Not in my known callers list, no answer'" was a great plan... that the folk in the room basically didn't understand (or even want me to voice, actually)... It's a shame that something like this wasn't created instead of shaken/stir. You could check the signature at any of the hops, start failing calls earlier as rates of completion didn't stay at some standard level. All sorts of options would be available, and really the callers could be identified (at least by endpoint) more quickly. oh well. glad we got shaken / stir though? :)