Los Angeles can now tax VoIP calls

Los Angeles has changed there tax code so that they can now tax VoIP calls. Well not a surprise that cities will start taxing VoIP many if not most already tax your cell phone charges. What I'm curious about is how they are going to do it. If you pay for the service or some part such as Skype Out/In minutes then they will have Skype/eBay adding the tax onto your purchase. But what about VoIP via your IM client AIM, MSN, Yahoo, Google Talk all have VoIP capabilities or even Skype to Skype these are all currently free. The updates to the tax code indicate the changes are broad enough they can also tax your Internet connection fees. But if they try to go after IM VoIP traffic how would they measure the usage and what would they base the charge on since it's free. Also Facebook and other Web 2.0 services are building in VoIP capabilities, so assuming cities try to get all the tax money possible how will they track all the new VoIP applications and there usage. Last I'm wondering where things currently stand at the Federal level, in the past taxing the Internet has been prohibited, has that changed and does that override state and local level taxing authority.