Below, please: s/such/VoIP filtering/ and it will be true. It do not depends of alghoritm you are using. Moreover, if you deploy such service, someone else can deploy VoIP which uses https tunnel to it, and you will not have any chances than to block total https traffic. It (such thing) can be more practical in corporate network (on gateway) or, yep, in case if third force (gov.) requires it and you really want to comply (not to show a compliance). But I just dropped an idea, not the whole solution.
However, attempting to deploy such in the presence of any competitive market would likely result in widespread customer migration. A government or assured monopoly could deploy it, though, and then charge a premium for those who want it removed (i.e. "premium data service") which conveniently happens to match their current voice tariffs...
/John