The basic idea is to implement a digital certification system to authenticate the origin of all converted packets allowed to pass at iptelephony gateways. Additional measures to aid in verifying geo-location of legitimate call centers such as PING/round trip signal time, packet loss, etc.
This solution is not a method for combating normal POTS calls. But the ability to successfully spoof caller identification through VOIP callsis what makes illegal call centers so appealing. Mandating such digital authentication be performed before ipcalls are converted POTS will make current call spoofing techniques obsolete for investigation and phone service records purposes.
This will not effect the experience of a call for a receiving user as all processing is done "server side", as it were.
I have no evidence that I can present on this forum.
But I may point out that the digital signature signing authentication scheme used for web traffic is very good method for verification which my proposed solution is slightly analogous to.