HiHi, good luck!

This is an very old requirement and was used in step-by-step systems in addition to trace back malicious calls (MCI) manually. It was also available on request on every normal extension to trace back malicious call via hook-flash, with the execption that the call was hold only until the end-office of the caller. This was already a lot of fun to implement in digital switching systems.

But we should leave "the church in the village", as we say in German. You simply cannot implement each feature of an old technology 1:1, even if you consider VoIP as fixed line replacement. This is also true for an other requirement: remote power-feed from the local exchange. This is also gone with ISDN-NT1 and DECT-phones. And I do not think that you requirement is available on cellular phones used by more than 50% of emergency calls already.

Since this is a social policy, the political forces will prevail over the technical difficulties. It is very difficult to predict how the public debate will shape up. As you indicate the public seem ready to accept restricted application for cellular phones; so the question is will they accept it for VoIP. The next lawsuit will generate the public debate.