No, it's simple. There is a commodity tax, like a value added tax on the service. When the company invoices for the service, be it from a phone bank in Bombay or down the street, a percentage goes to uncle sam, because this is where the service was purchased. And the commodity service can be at different rates for different commoditys. A simple customer support function is one rate and professional help like medical X ray analysis work could be another.
It's not that simple. Many offshored jobs are "blended" with jobs onshore. For instance, with call routing through IVRs, the next available agent is given the call, whether here or in India.
The cost of a project may involve programming offshore with project management onshore.
Your tariff scheme would be an absolute nightmare, both for the companies trying to track the source of their services, and the auditors who were trying to enforce compliance.